An all-solid-state doubly resonant intracavity sum-frequency continuous-wave 578 nm yellow laser with KTP type Ⅱ phase matching is studied. Two kinds of gain media, YbYAG and NdYAG, are used to generate spectral lines of the fundamental frequency at 1030 nm and 1319 nm. The 578 nm yellow laser output is achieved by intracavity sum-frequency with KTP type Ⅱ critical phase matching. It is found in the experiment that besides the output of the yellow laser at 578 nm, an output of sum-frequency yellow laser at 582 nm is found at the same time. By spectral analysis, the phenomena are ascribed to the oscillation of 1338 nm from the transition of R2→X3 in NdYAG crystal and the sum-frequency generation (SFG) between the wavelengths of 1030 nm and 1338 nm. When the pump powers of YbYAG and NdYAG are 10.3 W and 3.7 W respectively, the yellow laser with the output power of 55 mW is obtained. The power stability of the output laser is better than 4.7% within 30 min. By using a Glan prism, the polarization characteristics of the fundamental frequency light and the sum-frequency light are measured. The results show that for the two isotropic laser crystals, both the resonator structure and the azimuth angle of the SFG crystal have influence on the polarization characteristics of the corresponding fundamental frequency light, and both of them can change the polarization direction of the fundamental frequency light to the direction which benefits to the sum-frequency process.