In this paper,a novel heat sink which coupling natural convection and phase transition was designed to meet the cooling requirements of distribution level static synchronous compensator (DSTATCOM).The two inclined evaporation substrates can meet the requirements for the parallel operation of DSTATCOM and the working medium R245fa was driven by gravity and buoyancy in the heat sink.The influence of filling ratio and heating power on the thermal performance of the heat sink was investigated experimentally and theoretically.The experimental results showed that the filling ratio and heating power had a significant influence on the thermal performance of the heat sink.At the ambient temperature of 25℃,the heat sink could start up successfully at different heating power and filling ratio.Meanwhile,the heat sink achieved good heat dissipation performance as the maximum temperature of two evaporation substrates were 60.14℃ and 67.85℃ while the heating power was 3000 W and the filling ratio was 80％.The heat transfer resistance between two evaporation substrates and condensation substrates were 6.6× 10-3℃/W and 1.2×10-2℃/W at the same working condition.Additionally,the experimental results showed a good agreement with the calculated results.