Abstract: A flexible insulated wall solar greenhouse (FIWSG) can represent an upgraded improvement over conventional soil-walled ones. However, structural stability is often caused by insufficient stiffness, particularly towards larger spans, due to the replacement of thick soil walls by steel tubes with a large slenderness ratio. In this study, a Y-type braced multi-form arch was proposed in the FIWSGs, according to the mechanical properties of existing diagonal and strut brace systems. The stability was enhanced to reduce steel consumption via the local stiffness and internal force transmission paths of FIWSG. A static analysis was conducted on the Y-type braced solid-web FIWSG. Subsequently, a variable-stiffness truss arch in FIWSG was proposed to arrange the upper chord, lower chord, and web members according to the bending moment and deformation distribution diagrams. According to elastoplastic mechanics and nonlinear finite element (FE) theory, the Beam188 element was selected from its element library to simulate the greenhouse members using ANSYS software. The member base and the foundation were related to a fixed constraint. The arclength was employed to consider geometric nonlinearity in the descending branch of the equilibrium path. Additionally, the constitutive equation was adopted for the material nonlinearity parameters in a bilinear model, the von Mises yield, and the BKIN bilinear kinematic hardening model. Refined FE analysis models were established for FIWSGs with different brace types (no brace, diagonal, strut, 3/4-2.2Y-type, 27/40-2.2Y-type, 3/5-2.2Y-type, 3/5-2.7Y-type, and 3/5-3.2Y-type braces) and different arch forms (solid-web and truss). Elastoplastic stability analysis and parametric studies were conducted to compare the performances, including stability and steel consumption indicators, of the critical span applicable for arch selection in FIWSGs. The results indicated that: (1) The Y-type brace significantly improved the performance of the arch, compared with the brace types. Bifurcation heights and front roof support point position were compared to determine the 3/5-2.2Y- and 27/40-2.2Y-type braces. The maximum stability increased, with the 27/40-2.2Y-type brace superior overall performance. (2) The overall displacement of the 3/5-2.2Y-type brace was reduced by 10.3%, 19.8%, and 30.4%, compared with the scenarios with no brace, only diagonal brace, and only strut brace, respectively. The instability of the location gradually shifted toward the front wall. Arch stiffness and internal force transmission were obtained among the diagonal, strut, and Y-type braces. (3) The critical span between solid-web and truss arches was determined for the high stability and low steel consumption of FIWSGs with the spans of 10, 12, 14, 16, 18, and 20 m and different arch forms. Specifically, when the span was within 14 m, greater advantages were found for solid-web arches; on the contrary, more advantages were observed for truss types. The finding can provide the theoretical and technical reference for performance enhancement, arch selection, and engineering practice of FIWSGs.