By using DSC/TG, SEM, BET, particle size distribution and hydration temperature curve, the pyrolysis characteristics and mechanical properties of phosphorus recycled gypsum, desulphurized recycled gypsum and their changing mechanism were studied. The results indicated that the pyrolysis characteristics and mechanical properties of recycled gypsum were decreased, the twice dehydration temperatures of phosphorus recycled gypsum dropped 4℃ and 2℃, respectively, and desulphurized recycled gypsum both reduced by 2℃. The low decrease in strength and modest increase in water absorption were seen in phosphorus recycled gypsum, and the rate was 15%~30% and 22.40%, respectively. But the 45%~70% decrease in strength and 76.61% increase in water absorption were possessed by desulphurized recycled gypsum, which was much greater than phosphorus recycled gypsum, and approximately the same dehydration temperatures and mechanical properties were observed by the two recycled gypsum. The analysis has shown that the reduction in pyrolysis characteristics is due to the decrease of particle sizes in recycled gypsum. The main reason of the great difference of mechanical properties between raw gypsum and their recycled gypsum are the increase in specific surface area, decrease in particle sizes and the uneven distribution of recycled gypsum. And the poor crystal of phosphorus gypsum results in the low decrease rate of its recycled gypsum.