The characteristics of slag flow and heat transfer are significant for the stability and security of gasifier. A slag flow and heat transfer model under unsteady conditions was built to study the slag behaviors dynamic response with transient and periodic operating conditions change. The results show that slag behaviors changed at the beginning and tended to be stable as time increasing with the coal ash content and operating temperature transient changed. The trend of slag heat flux and SiC surface temperature are opposite of slag thickness. Moreover, periodic operating temperature variance was considered and the results show that the slag behavior changes were also periodic within a specific frequency range. The amplitude of the slag behavior change increased with the increasing amplitude of the operating temperature, but decreased with the increasing frequency of the operating temperature.