In this paper,under water temperatures 28. 5℃ ,the toxic effects of ammonia to Hemifusus tuba juveniles ( shells high 11. 3 ± 0. 11 mm,n = 30 ) were studied at different pH and salinity with ecological method. The result showed that salinity has significantly effect on the toxicity of ammonia. The toxicity of ammonia in water became stronger when the salinity was lower. When salinity was 16,19,23 and 28 respectively,the 96hLC₅₀ of total ammonia to H. tuba juveniles were 36. 5,43. 7,52. 6 and 58. 8 mg /L respectively,and equivalent the safe concentration (SC) of total ammonia to the juveniles were 3. 7,4. 4,5. 3 and 5. 9 mg /L,respectively. The toxicity of ammonia in water became stronger in higher pH than in lower. The 96hLC₅₀ and the SC of total ammonia to H. tuba juveniles were 58. 3,54. 5,50. 6,20. 2 mg /L and 5. 8,5. 5, 5. 1 and 2. 0 mg /L,respectively. The 96hLC₅₀ of total ammonia to H. tuba juveniles fell rapidly when pH was 8. 8,and toxicity of ammonia in water became 2. 9 times higher than in pH 7. 6.