The conventional thermal diffusion process for preparing P-N junction emitter of solar cells mainly consists of two steps: pre-deposition and high temperature drive-in. In this paper, high sheet resistance (Rs) emitters with a low surface phosphorus doping concentration were prepared on p-type multicrystalline silicon wafers by introducing a constant temperature process after the high temperature drive-in step. The influence of constant temperature on the Rs and phosphorus atom doping profile of P-N junction emitter was investigated. The results show that the Rs of the P-N junction with a certain constant temperature treatment can increase reversely. Correspondingly, the doping profiles tested by secondary ion mass spectrometry (SIMS) indicate that the phosphorus doping concentration at the surface of silicon was reduced when the constant thermal treatment at temperatures of 650 ℃ to 750 ℃ was carried out after the high temperature drive-in step. Compared with the conventional diffusion process, the Rs of the P-N junction with a constant temperature treatment at 700 ℃ for 15 min increased by about 3.2 Ω/□, and the conversion efficiency Eff of the corresponding solar cells increases to 18.69%, which is higher by 0.23% than that of the Baseline.