In this paper, based on the convective heat transfer and conduction principle, the thermal effect analysis model of the directly liquid cooled uniformly pumped thin slab laser is established. The approximate plane stress and the principle of minimum are introduced to describe thermal stress distribution in the thin slab. Firstly, the relationships between the flow velocities in different flow channel thickness values and the convection heat transfer coefficients and also the relationship between flow velocity and coolant temperature rise are studied. Moreover, the influences of different flow channel thickness values on temperature field and thermal stress distribution are analyzed. Finally, the variation trends of wave-front phase distortion with the change of heat power in the case of Zig-zag path and direct path are investigated, respectively. The results reveal that thicker flow channel can achieve stronger heat treatment effects in an appropriate range of the cooled liquid flow rate, and the thermal profile is symmetrical with respect to the center plane of slab. In addition, the longitudinal maximum temperature rise occurs in the outlet; the maximum stress distortions centralize on the both ends and partial sides of slab. It is worthy to mention that the one-dimensional temperature gradient and smaller stress form more probably for thicker flow channel., Furthermore, zig-zag path can alleviate obviously wave-front aberration due to thermo-optic effect. In this paper the thermal effect of the liquid direct cooled thin slab laser is investigated. The research results are beneficial to the design and optimization of the directly liquid cooled thin slab laser.