Abstract: In laser rod, the unevenly distributed pump-light caused the gain function vary over the medium, which could bring changes on the mode of the oscillated laser. A method was developed to estimate the influence degree of the non-uniform pumping light on the laser mode, by which the deviation of the oscillating field from its origin form could be given in terms of percentage. Based on it, a quasi-continuous three directional side pumped laser was discussed, in order to find the feature of the affection that the varying pumping light acted on the oscillating laser. Result shows that the changes of the oscillating mode brought by the pumping light have monotonic relations with the pumping power and the cavity loss, and the influence of pumping light on oscillating field only depends on its variation over vertical section, and is free from the distribution along the propagation direction of the oscillating laser. The magnitude order of the effect is obtained through numerical calculation, which is less than 4% when the pumping power is 2 000 W and is larger than 20% when the pumping power is 6 000 W.