Abstract: Internal oil in a hydropneumatic suspension is treated as a research object in recent thermodynamic studies; in fact,there should be a temperature gradient in the oil,and this may result in inaccurate predictions of the temperature change of the suspension. This paper outlines a method of dividing the internal oil into multiple regions to solve the problem. Firstly,a validation experiment was performed to test the temperature gradient in the oil. Then,a lumped parameter thermal model was established to describe the suspension used in the test. The most important is that mass transfer between the regions was taken into consideration in the model. Experimental results show that the temperature gradient exists in the internal oil. In other words,it is inaccurate to make the whole internal oil as a research object. Meanwhile,comparative results show that the proposed model can describe the temperature change regularity of the oil more accurately.