In an entrained flow coal gasifier, the slag viscosity property is of great importance to the slagging process. The slag viscosity is affected by the crystal mineral and the crystallization process has a time effect. The molten slag residence time in gasifier was calculated by slag flow model to estimate the isothermal time or crystal growth time during the cooling process. The residence times of tracer slag unit were calculated by the slag velocity. The residence time distribution (RTD) curve was obtained and the mean residence time was about 100-500 s in this study. Moreover, the molten slag mean residence time decreases significantly with increasing ash contents in coal, decreases slightly with increasing operating temperatures, and increases with increasing slag critical viscosities and temperatures. In addition, a plug flow reactor (PFR) series a similar laminar flow tubular reactor model was used to analysis the RTD curves of molten slag with different operating conditions.