A double-ring cavity fiber laser based on a high roundness microsphere cavity as the single-longitudinal-mode selector is proposed. By the improvement of the melt heating technology and surface cleaning process, microsphere cavities with high roundness and high Q are obtained. We achieve highly efficient coupling between the tapered fiber and the microsphere cavity by using a two-dimensional visual and three-dimensional high-precision coupling system. Through optimizing the polarization of incident light and the diameter of tapered fiber, we obtain whispering gallery mode (WGM) resonance spectrum with coupling efficiency of 99.5%, a 3 dB bandwidth of 1 pm and a side-mode-suppression-ratio (SMSR) of 14.5 dB. The proposed fiber laser produces narrow bandwidth laser spectra with a 3 dB linewidth less than 0.01 nm, a SMSR of 40 dB and a high stability without mode-hopping.