It is generally believed that the transverse mode instability (TMI) of fiber laser mainly comes from the thermal effect caused by the quantum defect of the pump source and the pump absorption of gain fiber. Theoretically, based on the analysis of the heat source in optical fiber, it was found that the thermal effect of induced mode instability mainly comes from pump absorption, followed by the quantum defect. And the simulation software SeeFiberLaser developed by the research group was used to verify the conclusion. The simulation results showed that the lower the pump absorption coefficient, the lower the maximum temperature and temperature gradient in the fiber, the more beneficial it was to restrain the formation of thermally induced refractive index grating and increase the TMI threshold. Experimentally, a co-pumped ytterbium-doped fiber laser oscillator with a core/inner cladding diameter of 30/400 μm was built and comparatively studied on TMI threshold characteristics of lasers pumped by laser diodes (LDs) with central wavelengths of 976 nm, 915 nm and 940 nm. The results showed that when 976 nm LD, 915 nm LD and 940 nm LD were used as pump sources, the laser TMI threshold was 279 W, 502 W and 697 W respectively, and the laser optical-to-optical (O-O) conversion efficiency was 67.7%, 61% and 63%, respectively. It was be found that the influence of the pump absorption coefficient on the TMI threshold was greater than that of the quantum defect on the TMI threshold, and the TMI threshold could be effectively increased by changing the pump wavelength and reducing the pump absorption coefficient. Optimizing pump wavelength, taking into account both quantum efficiency and pump absorption coefficient, was one of the important technical routes for fiber lasers to achieve high beam quality and high mode instability threshold.