A variety of structures are used in the design of the inner cooling systems for HV converter valves, while the problem of deposition on the pin-type grading electrodes is still frequently observed. In this paper, the deposition status in the cooling water ways of the quadruple valve towers, the double valve towers and water ways inside the valve modules are measured and counted on site. The effects of environmental factors including electric field parameters and fluid field parameters are analyzed. The distribution of electric field and electric current on the pin-type grading electrodes is numerically calculated. By means of simulating calculation, the effects of electric polarity and electric current on the deposition distribution are verified. The results show that the relative electric polarity of the grading electrodes is the dominant factor affecting the deposition degree. The deposition tends to occur on the grading electrodes located at the high electric potentials. A positive correlation between the electric current and the deposition thickness on the grading electrode is obtained. The morphology of the deposition is in significant consistence with the distribution of electric current on the pin-type grading electrode.