Due to the fast development of science and technology, people increasingly like to get information from the Internet rather than newspapers and magazines. However, recently, this accessible online information has been filled with enormous negative news or neutral news with negative headlines. Therefore, the current study conducted three experiments to examine how negative online news affects social trust and helping behavior. The results of experiment 1 showed that individuals were inclined to demonstrate an attentional bias on and perform a preference for the negative news during the eye-movement task and indicated that individuals were easily affected by negative online news compared with positive online news. Based on experiment 1, experiment 2 used the guiding effect of online news and found that relative to some readers who were presented with positive news, others who read negative news showed less helping behavior, and this relationship was completely mediated by social trust. In experiment 3, we changed the headline of every neutral news story into two versions, one with a neutral headline and another with a negative headline, and found more negative cognition, lower social trust, and less helping behavior when individuals read negative headlines. Results of the current study supported the general learning model and the social cognitive theory, which showed that negative news had an impact on individuals' cognition, such as social trust, and then influenced their helping behavior. In particular, negative headlines led to a severely negative effect on social trust and helping behavior.