Using matrix method, we analyzed the high power c.w. laser of several YAG:Nd rods in series. The thermo-focal lengths and dimensions of these laser rods may be different. By using the electronic computer, the parameters describing operating characters of laser (especially mode-volume match parameter) have been obtained as a function of the geometric construction of resonator and the thermo-focal lengths of laser rods. In addition, experiments of laser oscillating were carried out on a laser with two rods of different dimension. The experimental results are quite agree with the theoretical analysis.