The unique two-dimensional electron gas(2DEG) structure in GaN-on-Si planar GaN enables excellent performance, but it also brings negative effects such as current collapse, threshold voltage offset and on-resistance(Rdson) shift. This paper investigates the effects of voltage, temperature and switching mode(hard/soft switching) on the Rdson of GaN power devices, specifically for 650V commercial GaN power devices with different structures from GaNsystems, Nexperia and Visic. This paper also presents a GaN on-resistance test platform to observe the Rdson shift process within 10 hours. The results show that Rdson shift is a common problem for all three GaN power devices under test. The degree of on-resistance shift is positively correlated with the drain voltage and negatively correlated with the junction temperature. In addition, soft switching can effectively suppress Rdson shift by avoiding simultaneous high voltage and high current.