Cascode GaN has the advantages of high current carrier mobility, low conduction resistance and low parasitic capacitance and it can be applied to Sine amplitude converter with high frequency properly. In addition, Cascode GaN driver circuit is fully compatible with the drive of silicon transistor. It can reduce switch loss, improve efficiency, and generate less heat. However, there are few articles analyzing the influence of cascode GaN for sine amplitude converter driver circuits currently. We used cascode GaN as switching transistor and used isolated driver chip in driver circuit. In actual application, the driver voltage of half-bridge upper arm fluctuated. This paper aims to analyze the reason of fluctuation in the actual circuit. Equivalent body diode of cascode GaN transistor can be replaced by a low-voltage silicon diode in series with a resistor. The reverse recovery time of Cascode GaN is not short enough, so that fluctuations occurred. An analysis of the driver and transient circuit shows that voltage oscillation between drain and source makes charge pump voltage unstable, which in turn makes driving voltage fluctuate and three methods are given in this paper. Finally, the effectiveness of the methods is analyzed in an actual circuit.