The formation of hydrothermal reaction products in fly ash-lime system by autoclaving were investigated by X-ray diffraction analysis ( XRD) and thermal analysis. HCl selective dissolution method was used for quantitative analysis of fly ash reaction degree. Combined with quantitative analysis of lime reaction degree, the content of hydrothermal reaction products were estimated. Results showed that dicalcium silicate hydrate (C2SH),tricalcium aluminate hydrate(C3AH6) and hydrogarnet (C3ASH4) were synthesized by hydrothermal reaction in fly ash-lime system under autoclaving. With the increase of autoclaving parameters from 4 h an 120℃ to 6 h and 160℃,the content of C2 SH and C3 ASH4 increased from 6. 94％-17. 69％ to 13. 87％-17. 46％ and 2. 91％-5. 99％ to 15. 03％-16. 08％ respectively,reaction degree of fly ash increased by 50％,reaction degree of CaO increased by 32. 59％ and free CaO ( f-CaO) mass concentration reduced by 46. 49％ in autoclave sample with fly ash ratio of 78％. Experimental results showed that the optimal ratio of lime was 22％,the appropriate autoclaving parameters were 140℃ and 8 h or 160 ℃ and 6 h or more,respectively.