A continous-wave (CW) 500.9 nm laser using intracavity sum-frequency mixing in a doubly resonant configuration is reported. The laser has two sub-cavities. A Nd:YAG crystal and a Nd:YVO4 crystal are pumped by two laser diodes respectively in two sub-cavities. In the two cavities wavelengths of 946 nm from Nd:YAG and 1064 nm from Nd:YVO4 are chosen to be mixed into 500.9 nm laser. A composite Nd:YAG crystal is used to deduce the thermal lens effect under high power pumping, and cavity design is optimized considering the thermal lens effects. Better matching of the modes for the two wavelengths is obtained. In the overlapping of the cavities, sum-frequency mixing is generated with a type-Ⅱ critical phase matching KTP crystal. An output power of 730 mW at 500.9 nm is obtained at the incident pump power of 17.8 W for the Nd:YAG crystal and 10.6 W for the Nd:YVO4 crystal. The analyses and experiment show that the doubly resonant intracavity sum-frequency mixing is a useful method to obtain 500.9 nm laser efficiently.