The gravitational constant may be constantly changing. Although the relevant evidence is not very sufficient at present, from the existing experimental results, the gravitational constant is the least accurate constant among all the constants. This also shows that the gravitational constant may change with the change of space-time. This paper attempts to explore the possible reasons for the change of the gravitational constant. This work is also based on the results of some of my previous works. But this paper mainly discusses on the basis of the fluid model of dark matter. According to the fluid model of dark matter, dark matter in the entire universe is a gaseous fluid state. And this fluid naturally has various thermodynamic properties. Which involves pressure, temperature and volume, and so on. If the temperature or pressure of the dark matter fluid is affected, the viscosity coefficient of the dark matter fluid will change. In the dark matter fluid model, it is assumed that the viscosity coefficient of the dark matter fluid is a parameter that is completely consistent with the gravitational constant. Therefore, if the viscosity coefficient of the dark matter fluid increases or decreases, it will directly cause the gravitational constant to increase or decrease in this part of space-time. This paper analyzes the difference in temperature in different space-time locations of the solar system, and points out that in the location close to the sun, due to the relatively high temperature, the temperature of dark matter in this part of the natural world will also be relatively high. If the dark matter fluid is a gas, the higher the temperature, the higher the viscosity coefficient. Combined with some previous analyses, we can see that the dark matter fluid should mainly exist in the form of gas. This is because from the results of these analyses, the gravitational constant should be larger the closer you are to the sun.