Physical mechanism of the sideband asymmetry is theoretically analyzed in the fiber ring lasers,A passive mode-locked erbium-doped fiber ring laser is constructed in the experiment,The obvious asymmetry of the spectral sidebands in an L-band is obtained by adjusting polarization controllers. The asymmetry of sidebands can be observed in two aspects:on the one hand,the asymmetry of the power between the positive and the negative order numbers is obvious,which can be observed that the intensity of the positive second order is higher 14.28 dBm than that of the negative same order;on the other hand,the sideband asymmetry numbers from the positive to the negative orders are also obvious,the number of the positive orders is more 5 numbers than that of the negative. The sideband asymmetry is useful for the research of eliminating the sidebands and acquiring the ideal soliton pulses and so on.