Anthraquinone working solutions used in hydrogen peroxide manufacture were selected as the test system to investigate the flow pattern transition behavior of foaming system in the trickle bed reactor (TBR) with the method of measurement and analysis of the instantaneous pressure drop fluctuations. The boundaries and the corresponding flow map for the foaming system were established for TBR packed with different size particles. It was found that the size of particles shows significant effect on the flow regime distributions. A theoretical model proposed by Cheng and Yuan and an empirical correlation by Bartelmus and Janecki were used to predict boundaries of the flow pattern transition for anthraquinone working solutions. Comparison between estimation and experimental results showed that the predicted boundaries of Cheng and Yuan theoretical model are consistent with the observations in the present work.