Abstract: The non-contact electric eddy current displacement sensor system was applied to real-time monitor the preform thickness change throughout the vacuum assisted resin infusion (VARI) process. The compaction behavior of the preform during the VARI process was discussed. And the effect of the post-filling time on the preform thickness and the fiber volume fraction was studied. The result indicates that the compaction behavior of the preform can be divided into three stages: the preform thickness increases rapidly when the resin is infused; the perform thickness is almost not changed during the post-filling; the preform thickness decreases rapidly and drives to a stable value after that the resin-inlet is clamped. The final part thickness varies with the post-filling time, and the variation tendency is similar to a sinusoid. The fiber volume fraction is the lowest and decreases 1.7% compared with that without post-filling, when the post-filling time is about 10 min. And the fiber volume fraction is the greatest and increases 1.6% compared with that without post-filling, when the post filling time is about 40 min.