Hydraulic valve is used as an important hydraulic component, playing a vital role in fluid power transmission and control systems. When the phenomenon of sticking of valve core occurs, it may seriously decrease the accuracy and sensitivity of the hydraulic valve. Micro-deformation is one of the most common faults causing the valve core sticking, which is due to the thermal load. Therefore, studying the mechanism of valve core sticking caused by thermal fluid is of great significance. In this paper, the fluid dynamics in hydraulic valves and the temperature characteristics of valve core are analyzed based on the thermal-fluid-solid coupling method. Results show that the jet angle (θ) will decrease with the increase of opening degree (k), and the maximum velocity and temperature increase with the increase of the opening degree. By loading the temperature field into the thermal analysis as a boundary condition, it shows that the temperature of the U-throttle groove increases with the increase of the opening degree. In addition, with the increase of opening degree, the maximum thermal deformation increases and the contact pressure between valve core and valve body also increases. This work has a certain reference value for researching and improving the phenomenon of valve core stuck in hydraulic valve.