The thermal expansion of the object reflects the property of the material itself, when solid is being heated, linear expansion is usually described as a one-dimensional changing in the length. Measurement of linear expansion coefficient of materials was not only important in their developments, but also one of the most important standards in choosing them. Combining Doppler effect with laser heterodyne technology, a novel measurement method of multi-beam laser heterodyne for metal linear expansion coefficient was proposed, which was converted into the measurement of the length variation of linear expansion coefficient. Based on Doppler effect of oscillating mirror, the information of length variation was loaded into the frequency difference of the multi-beam laser heterodyne signal by the frequency modulation, and many value of length variation could be acquired simultaneously after the multi-beam laser heterodyne signal was demodulated. Processing these values by weighted-average, length variation of the sample versus temperature could be obtained accurately, and eventually the measuring of linear expansion coefficient of metal was improved. Simulations for linear expansion coefficient of metal rod under different temperatures had shown that the relative measurement error of this method is just 0.1%. The measuring accuracy was improved by one order of magnitude compared with traditional measurement methods.