The effective surface charge of colloid particles is an important parameter that determines the colloidal properties. However, it is still unclear whether the solvent environment (such as the electrolyte concentration) can affect the effective surface charge. Due to complicated effects relevant to the effective surface charge, such as the exchange of dissociable ions between the electrolyte and surface groups of polystyrene particles, the coupling effect of incomplete ionization of the surface groups of the particles and the adsorption of ions by colloidal particles, etc., it is rather difficult to accurately measure the surface charge and understand the mechanism of charge variation with solvent environment. To solve this problem, we measure the conductivities of polystyrene colloidal particles of carboxyl groups and sulfonic acid groups at various particle number densities and HCl concentrations. Since the cations generated from the two kinds of particles and HCl solution are all H ⁺cations, the surface charge can be obtained by the conductivity-number density method (migrant method), no matter whether the cation exchanges occur between ionized positive ions of the electrolyte and colloidal particles. Based on the experimental results, the influences of HCl concentration and particle number density on the surface charge of colloidal particles are detected, and the reasons of the influence are analyzed. It is found that the change of the surface charge of the particles of carboxyl group with HCl concentration is faster than that of sulfonic acid group with the HCl concentration. For the same electrolyte concentration, the effective surface charge of carboxyl modified colloidal particles is related to the particle number density, while the charge of sulfonic modified particles is not. Considering the fact that the sulfonic acid group and carboxyl group are strong and weak acid groups respectively, the ionization of H ⁺cations of the two different groups have profound influences on the cation replacement process, and affect the trend of the curve of the conductivity-particle number density. This effect further results in different change tendencies of effective surface charge with HCl concentration and particle number density. According to the theoretical model as described in this study, all experimental results are well explained. The mechanisms described in this article will be useful for stating the influencing factors of the surface effective charge, and the application of the effective charge to different phenomena relating to interparticle interactions with different parameters of solutions.