The testing mass is the core sensor for measuring the spatial gravitational waves. The high-energy cosmic ray particles penetrating the outer structure of the spacecraft result in the electrical charges on the testing mass. The Coulomb force produced by the charges on the surrounding conducting surface and the Lorentz force generated by the motion through the interplanetary magnetic field will exert a serious influence on the geodesic motion of the testing mass. In this paper are investigated the process and mechanism of charging the testing mass by high-energy particles from different cosmic rays through using the Monte Carlo simulation method. It is concluded that the charging rate gradually increases with the decrease of cut-off energy under the same energy spectrum. The positive charging rate (elementary charges per second) in the years of minimum solar activity is predicted to be 39.5 +e/s, and the protons account for approximately 83.16% of the total quantity of galactic cosmic rays. The positive charging rate of the testing mass during the years of maximum solar activity is about 12.5 +e/s, and the charging rate of the testing mass of the worst solar energetic particle event in 1989 is about 120700 +e/s. The charging rate of the components of the galactic cosmic ray depends on the deposition of primary particles of each component in the testing mass during the years of minimum solar activity, with primary particles accounting for 73% of the total charging rate. The charging contribution of protons in years of minimum solar activity is mainly in an energy range of 0.1–1 GeV, accounting for about 65%. The research results can be used to assess the charging patterns of test quality on-orbit charges and provide a basis for designing the charge management and on-orbit work.