Abstract: In the process of producing phenyethanoid glycosides from Cistanche deserticola,large quantity of mannitol was discarded along with the carbohydrate solution.To obtain mannitol from these discard solution,a crystallizing process was carried out.The effects of three main influencing factors( including density of carbohydrate solution,crystallizing time and temperature)on both recovery rate and purity of mannitol were tested. Results of single- factor experiment showed that three appropriate operational parameters were: 1.21~1.27 g/mL,24~30 days and-10 ~0 ℃. Response surface methodology was used to optimize crystallizing process.Results illustrated that the optimal operational conditions were as follows: density of carbohydrate solution was 1.24 g/mL,crystallizing time and temperature were 28 days and 0 ℃.Under these conditions,the recovery rate and purity of mannitol were 70.04% and 87.17%. Therefore,discarded carbohydrate solution produced in the process of separating phenyethanoid glycosides from Cistanche deserticola can be used to separate mannitol.Besides,the models obtained in this study can be used to optimize the crystallizing process of mannitol.