Uncertainty is the most prominent feature of current work tasks, and also one of the important factors affecting employees’ work mental state. According to the Job Demand-Resource model, two opposite psychological states of thriving at work and job burnout were selected as the outcome variable, and challenge_x005fhindrance stress was used as the mediating variable to study the effect of task uncertainty on employees’ work mental state. The study found that task uncertainty has a double-edged effect on employees’ work mental state. Task uncertainty directly boots thriving at work, and indirectly reduces thriving at work and aggravates job burnout through the masking effect and intermediary effect of hindrance stress. Further studies have found that the positive effects of task uncertainty are caused by the large variability of the task, and the negative effects are caused by the low analyzability of the task. This conclusion provides guidance for management practice to cope with task uncertainty properly.