The COVID-19 pandemic has caused considerable concerns on the ‘infodemic’, which is characterized by an overabundance of information, particularly false and misleading information.  Despite the growing literature, how the infodemic can run in parallel with the pandemic has received little theoretical attention from a risk communication perspective. This study adopted the social amplification of risk framework to delineate the process of how risk information from social media can result in individuals’ misinformation sharing through heuristics processing of information. The amplification role of social media trust was also examined. It was hypothesized that people would feel more negative and unsatisfied after exposure to risk information from social media when they have a low trust in this information source. In contrast, when holding a high level of trust in social media, people would recall more misinformation exposure. Both negative feelings and misinformation recall would serve as heuristic references for misinformation sharing decisions. Results from an online survey of 1,488 Chinese Internet users supported the hypotheses and revealed that exposure to social media risk information induced misinformation sharing through affect heuristics (i.e., negative affect) and availability heuristics (i.e., misinformation exposure). Importantly, both high and low level of trust in social media amplified misinformation sharing. While low level of trust strengthened the association between social media risk information and the affect heuristics, high level of trust strengthened its association with the availability heuristics, both of which could led to more misinformation sharing. Our findings suggest a complex picture of the role that social media plays in a health crisis like pandemic. While social media is an important risk information source, individuals should maintain a middle level of trust, rather than blind trust nor distrust, in social media, being open while critical to risk information from this source during a health crisis.