Abstract: In order to study the preventive effects of toltrazuril suspension on Cystoisospora suis infection in piglets, in the test, coccidia oocysts were isolated and purified and expanded to establish a model of artificial infection of piglets with coccidiosis; the optimal infection dose was determined by measuring the weight gain, diarrhea, and the number of fecal coccidia oocysts（OPG） per gram of piglets. 90 piglets at 3 days of age were randomly divided into infection control group, reference group and test group, with 30 piglets in each group. At 6 days of age, piglets were dosed with 5% toltrazuril suspension in the reference group and the test group respectively, and 24 h later, the three groups were orally administered the optimal infective dose of sporulated oocysts respectively. The positive rate of coccidia oocysts and the amount of infection in each group were determined, and the fecal trait score and diarrhea rate, feed-to-weight ratio and mortality rate, and intestinal mucosal lesions were calculated. The results showed that the optimal infective dose was 1×10~4 sporulated oocysts. The positive rate of coccidia oocysts in the feces of the test group and the reference group was significantly lower than that of the infection control group from the 3rd to the 11th day（P＜0.01）, and the difference between the test group and the reference group was not significant（P＞0.05）. The reduction rate of coccidia oocysts per gram in the feces of both the test group and the reference group reached 100%. The diarrhea rate and fecal trait score on day 7, 14 were significantly lower in the test and reference groups than in the infection control group（P＜0.05）, and the difference between the test and reference groups was not significant（P＞0.05）. The average daily weight gain of piglets in the test group and the reference group was significantly higher than that of the infection control group from 8 to 14 days（P＜0.05）; the difference between the test group and the reference group was not significant（P＞0.05）. The feed-to-weight ratio was lower than that of the infection control group at the same time; the mortality rate was 3.3% in the test group and 0 in the reference group. There were no obvious lesions in the small intestinal mucosa of the test and reference groups, and the mucosal lesion scores were significantly lower than those of the infection control group on day 7,14（P＜0.05）, and the differences between the test and reference groups were not significant（P＞0.05）. The results indicated that the 5% toltrazuril suspension test product could effectively prevent Cystoisospora suis disease, and its clinical effect was comparable to that of the 5% toltrazuril suspension reference product.