Charitable giving, as a vital mechanism of the “third distribution”, plays a key role in advancing common prosperity. However, the total volume of social donations in China has shown consecutive declines in recent years, and credibility scandals involving certain charitable organizations have severely eroded public trust, highlighting the central role of trust in the philanthropic sector. Although existing research has explored various factors influencing charitable giving, two main limitations remain. First, there is a lack of in-depth empirical analysis on how social trust specifically affects donation decisions, particularly regarding the differentiated effects of different types of trust. Second, few studies systematically compare the heterogeneity in charitable behaviors between system insiders (e.g., public sector employees) and members of the new social stratum, who are key drivers of the market economy, within China’s unique institutional context. Using data from the 2024 “Survey on Living Conditions of Residents in Megacities in the New Era”, this study draws on Weber’s typology of trust to distinguish between generalized trust (universal trust in social members) and specific trust (trust in charitable organizations). A binary logistic regression model is employed for the empirical analysis, which yields three core conclusions. First, social trust has a significant positive impact on charitable giving behavior. Second, the participation rate in charitable giving among system insiders is significantly higher than that of members of the new social stratum. Third, different types of social trust exhibit differentiated driving mechanisms for the two groups: the donation behavior of the new social stratum is primarily driven by generalized trust, whereas that of system insiders is mainly driven by specific trust. The theoretical contributions of this study are as follows. It is the first to systematically compare the charitable giving behaviors of the new social stratum and system insiders in the context of institutional differences, revealing the group heterogeneity in how social trust influences charitable giving. It also differentiates the impact pathways of generalized trust and specific trust, deepening the understanding of the mechanisms of charitable giving in the Chinese context. Furthermore, it provides empirical evidence for the “united front work” concerning the new social stratum and for the development of the philanthropic sector. Policy implications include: enhancing the transparency of charitable organizations to rebuild public specific trust; promoting the reorganization of the new social stratum to broaden their channels for social participation; adopting differentiated incentive strategies, such as strengthening social responsibility education for system insiders and optimizing tax incentives for the new social stratum; and constructing a collaborative governance framework for philanthropy that involves the government, the market, and society.