Abstract: Substrate block seedling cultivation has been one of the most effective ways in high-speed mechanical transplanting, due to the short seedling period, the low root injury rate, as well as the low loss rate of water and fertilizer. Among them, the straw composts can be expected to serve as the seedling substrate instead of peat. The high value-added utilization of straw can also be enhanced in the field of substrate block seedling transplanting. Therefore, it is also required for the green utilization and molding technology of straw substrate. The molding quality of substrate blocks shared an important influence on their seedling cultivation and mechanical transplanting. Among them, the binder has been one of the key influencing factors in the molding of substrate blocks. This study aims to explore the effect of the binders on the molding quality and seedling performance of substrate blocks. Six binders included the polyacrylamide (PAM), xanthan gum (XG), sodium carboxymethyl starch (CMS), sa-son seed gum (SSG), guar gum (GG), and sodium polyacrylate (SP). Three amounts of 1%, 3%, and 5% (mass ratio) were also added. A systematic investigation was made to explore the molding quality (including molding expansion rate, dimensional stability, scatter rate, instant water expansion properties, and destructive load) of the substrate blocks, chemical microscopic components, and the emergence of Brassica chinensis L. seedlings. The binder was optimized with the higher compressive properties, the lowest cost, and the least addition amount, in order to maintain the dimensional stability and seedling requirements. Fourier transform infrared spectroscopy (FTIR) analysis showed that the unique chemical groups of different binders dominated the bonding strength and water absorption stability of the substrate blocks. According to the evaluation of molding quality and seedlings emergence, the XG was preliminarily selected as the binder of the straw compost for the seedling substrate blocks, where the amount range of addition was optimized to 1%-3%. Finally, the effects of the XG addition amount and molding pressure (550, 750, and 950 N) on the molding quality (destructive load, dimensional stability, and scatter rate) of the substrate blocks and the growth behavior of Brassica chinensis L. seedlings were investigated. The results showed that the XG addition amount and molding pressure shared a significant influence on the anti-destructive load and scatter rate. The molding quality of the substrate blocks was proportional to the addition amount of the binder and the molding pressure. The substrate blocks performed better molding stability under the molding pressure at 950 N, according to the comprehensive evaluation. The seedling cultivation experiments showed that the straw compost substrate blocks added with XG had no adverse effects on the growth of Brassica chinensis L. seedlings. The straw compost substrate blocks shared more leaf numbers and a larger development degree than the peat substrate blocks. The reason was that the straw compost also contained more nutrition for seedling production. The same seedling performance was observed in the peat substrate blocks and straw compost substrate blocks that were prepared with 3% XG under the molding pressure of 950 N. The findings can also provide a theoretical basis for replacing the peat with the straw compost molding in substrate blocks seedlings.