Abstract: The self-developed real-time online monitoring system has been applied to monitor the polymethyl methacrylate (PMMA) bulk polymerization under different polymerization temperatures online in real time. And then a real-time online curve of "virtual signal- reaction time" can be made according to the results. Through the changes in the curve, the four stages of bulk polymerization process, the induction stage, the early stage, the middle stage and the later stage can be clearly observed. According to the characteristics of the "conversion rate-reaction time" S-shaped curve of the bulk polymerization, the curve of "virtual signal-reaction time-conversion rate" and polymer conversion rate calculation formula were constructed. The real-time online monitoring and prediction of PMMA pre-polymer were proposed. This method is a new online real-time analysis method with many advantages, such as simple operation, strong real time, less influence caused by external factors, stable performance etc.