An in-situ interferometric technique was established to investigate the gelation process of poly(acrylic acid) (PAA) gel. The basic principles of the in-situ interferometric technique is illustrated. It can provide sufficient information for a non-destructive and successful investigation throughout the whole gelation process. Moreover, the effect of the photoinitiator concentration on the gelation process was studied. The polymerization rate of AA increases with increasing the concentration of the photoinitiator. The error caused by thermal effects during the gelation process can be neglected.