The invention publishes a low noise harmonious mono-frequent optical fiber laser which is used in the entire optical communication and its test system, aimed at solving the density change of the free current carrier in the source area of the semiconductor laser launch process, caused the twitter effect to cause the line-breadth phenomenon, could not satisfy the question that the high speed large capacity development request of the high speed wavelength division multiplying system and the optical communication. The laser contains the pump source, the optical coupler, the light isolator, the invariable light attentuator, the wavelength division multiplexer, the erbium ytterbium double mix optical fiber, the light circulator, the optical fiber polarizer, the er-doped optical fiber and the harmonious fiber grating. The test system is composed of the optical fiber laser, the light modulator, the actuation module, the error code analyzer, the light oscilloscope or the error code analyzer, the receive module and the light attentuator. The laser has mono-frequent, low noise, low twitter; the output is stable; suitable to use in the high speed long distance optical communication and take as the photo source in the optical fiber sensing; the test system may carry on the test analysis to laser's high speed electric light modulating characteristic.