In addition to basic roles such as storing hydraulic fluid, separating metal particles, releasing gas and water mixed in the hydraulic fluid, the hydraulic reservoir is also an important heat dissipation component of the open-loop hydraulic system. Currently, there are various recommendations regarding the selection of calculated values for the average heat transfer coefficient of the hydraulic reservoir, which has caused significant challenges for researchers in calculating the heat dissipation of an open-loop hydraulic systems. This may also lead to computational errors in previous studies. In this paper, the research team presents the results of developing a mathematical model and a Matlab-Simulink block diagram to simulate and calculate the dependency of heat flux (Qb) and average heat transfer coefficient (α₀) of the hydraulic reservoir of the Komatsu PC750-7 hydraulic excavator on the changes in temperature of hydraulic fluid and ambient temperature. The study demonstrates that the average heat transfer coefficient is not a constant but a nonlinear function dependent on the temperature of the hydraulic fluid and ambient temperature. This provides a reliable method for calculating and selecting the average heat transfer coefficient of the hydraulic reservoir.