Abstract: In-beam PET imaging provides a noninvasive method to monitor the dose distribution in real-time for particle therapy such as carbon ion therapy and proton therapy. This work designed in-beam PET prototypes based on free PET including cylindrical, open-PET, C-shaped, dual-head plate and rectangular PET scanners and simulated in-beam PET imaging based on these scanners in GATE, a Monte Carlo simulation platform. The scanners were composed of 32 detector modules through free combination and had similar field of view, which could be used for dose monitoring in carbon therapy for the different part of human body and small animal. Each detector module was composed of LYSO array coupled with SiPM. The crystal array contained 20×20 crystals measuring 1.5 mm × 1.5 mm × 10 mm with 1.6 mm pitch. A PMMA phantom was irradiated by the periodic carbon ion pencil beam with an energy of 230 MeV/u and was monitored by the in-beam PET imaging using GATE macros. PET images of the generated positrons were reconstructed by using a MLEM algorithm. Results showed that the shift of image’s peak recreated the variation of positron activity, making it feasible for range verification in the carbon ion therapy by use of free PET scanners. This work compared the reconstructed images between the experimental measurement and simulation based on the dual-head plate scanner containing 8 detector modules. Both peak positions of experimental and simulated images were at 27 mm depth, which validated the feasibility of free PET prototypes for range verification and dose monitoring in carbon ion therapy.