The development of self-frequency doubling lasers has been significantly limited owing to the thermal effect. To reduce the thermal effect of lasers and obtain higher beam quality yellow laser output, a Yb∶YCOB self-frequency doubling crystal yellow laser pumped by a 1020-nm laser is proposed. The influence of the thermal lens effect, because of the self-frequency doubling crystal-absorbing pump light energy, on the phase matching condition during the process of optical frequency doubling is explained on the basis of a theoretical analysis. Furthermore, the analysis also yields an explanation for the improvement in the quality of the 1020-nm single-band pumping mode beam, as compared with that of the traditional 976-nm laser diode pumping mode. A 570-nm yellow laser with higher beam quality is obtained through experiments. The beam quality factors correspond to Mx2=1.41 and My2=1.87. The results indicate that the quality of the outgoing beam at a wavelength of 1020 nm is significantly higher than that at 976 nm, and the spectrum width is narrower. The results of the study play a guiding role in reducing the thermal effect of Yb∶YCOB self-frequency doubling lasers and obtaining high-power yellow lasers with higher beam quality and narrower spectrum width.