The fundamental wavelengths are oscillated at 1064 nm and 1342 nm in a diode array pumped Nd∶YVO4 laser. Continuous wave (CW) sum frequency generation (SFG) of 593.5 nm is achieved by placing the nonlinear optical crystal LiB3O5 (LBO) cut as type-I phase matched in the cavity. Noise characteristics of CW intracavity SFG laser is measured and analyzed in different experiment conditions. The multi-longitudinal modes structure and transverse modes pattern of SFG laser is observed in high and low noise operation by Fabry-Perot (F-P) scanning interferometer and beam profile instrument, respectively. The results show that noise characteristics of SFG laser is closely related to the multi-longitudinal modes structure which is associated with the pumping power and the resonator cavity perturbation. Compared with unstable multi-longitudinal modes structure, the output laser with stable multi-longitudinal modes structure is lower noise operating. In addition, higher-order transverse modes pattern can also cause the increase of laser noise.