Extensive scholarship has examined how government surveillance affects citizens’ political participation, yet the mediating and moderating factors shaping this relationship remain insufficiently explored. This study investigates government-operated online surveillance in Hong Kong, a region that has experienced significant political transformation following the implementation of the National Security Law in 2020. Through an online survey (NN = 310), we examined how perceived surveillance influences online political participation of varying visibility levels, revealing a more complex picture than previously understood. While surveillance demonstrated pervasive chilling effects across both high-visibility and low-visibility activities, our path analyses uncovered distinct underlying mechanisms. Notably, fear emerged as a positive mediator between surveillance and political participation, actually stimulating citizens’ expressive and information-seeking behaviors. Additionally, trust in digital platforms moderates surveillance’s impact, particularly buffering its effects on low-visibility political activities. These findings contribute to surveillance studies by highlighting the dual nature of surveillance effects, where it simultaneously constrains and motivates political participation through distinct psychological mechanisms. Moreover, the study illuminates how digital platform characteristics and trust influence political activities online, offering insights into citizens’ adaptive strategies under increasing surveillance pressure.