The effect of cross-section shape and geometrical parameters of the channel on heat transfer and thermal deformation of the microchannel water-cooled mirror is studied by analyzing the temperature field obtained by using finite volume method to solve the three-dimensional steady laminar flow and heat transfer equations, and the deformation obtained by coupling the temperature field to ANSYS software. Three different cross-section shapes of microchannel, i.e., rectangle, trapezoid and circle, are investigated in this paper. Average heat transfer coefficient, temperature inereasing and thermal deformation of mirror of each shape examined with three geometrical dimensions, are simulated. It is found that for the same channel, temperature distribution is not symmetrical, the highest temperature moves to the downstream, the heat transfer coefficient of each interface is also different, the heat transfer coefficient of side wall is the biggest, heat transfer coefficient of other walls decreases along the water flow direction. For the mirror using the same shape microchannel and hydraulic diameter, the heat transfer coefficient decreases as the distance of the channel from water inlet increases. The average heat transfer coefficient and thermal deformation are related to hydraulic diameter and channel shape. Reducing hydraulic diameter may help to induce high heat transfer coefficient. The trapezoid channel mirror has the smallest thermal deformation among the three shapes channel mirrors. At the condition of heat flux of 14730 W/m2, hydraulic diameter of 239 μm and water inlet velocity of 2.54 m/s, the thermal deformation of the mirror with trapezoid channel is only 0.016 μm.