The recycled gypsum sample with naphthalene type（FDN） superplasticizer added was prepared under the condition of laboratory simulation. The sample was dehydrated by calcination to hemihydrate and its mechanical properties were tested. The influence of FDN on the performance of recycled gypsum and its mechanism were investigated by BET, DSC/TG and SEM EDS. The results show that the mechanical performance of plaster of Paris（PONP） and its recycled plaster（R NP） with FDN are increased compared with the plaster of Paris（POP） and its recycled plaster（R P） without FDN. The strength increasing rate of R NP is much higher. The strength of R P and R NP both are decreased in comparison to POP and PONP, while the strength of R P is dropped much compared with that of R NP. The analysis results indicate that the FDN still works in R NP due to the undecomposition and adsorption of FDN during the recycled gypsum calcination, and the greater fineness of R P also contributes to the dispersion of FDN, leading to the complete development of crystals and their close overlapping, which results in the decrease of porosity and average pore size. This is the very reason that superplasticizer reinforces its hardened R NP.