Abstract: The influence of the thermal effect of the active medium on the laser oscillating modes distribution of end-pumped solid-state lasers is theoretically analyzed. With the fiber coupled diode lasers which end-pump the Nd∶YAG laser, the influence of the thermal effect on laser oscillating modes was studied. It proves that TEM00 mode volume becomes larger when the pumping power increases，and TEM00 mode becomes dominant in the mode competition. As the pumping beam departures from the geometry cavity axis, the light spot center of TEM00 mode moves to the axis of the pumping beam and the quantity of the beam wandering becomes linear with that of the pumping beam axis. The results of the experiment agree with the theory.TEM00 mode output in which the ellipticity is 0.98 and M2 is 1.01 was obtained.