In-beam PET imaging is an advanced image-based method to verify the proton beam range for proton therapy by measuring proton-induced positron activity distribution and activity range. This study investigates the feasibility, accuracy and precision of the activity range measurement with a high-performance compact PET prototype system for in-beam PET imaging. An experiment with a homogeneous PMMA phantom and several Monte Carlo simulation studies are conducted. The results have shown that the prototype PET can provide reasonably good images for the activity range measurement even with low count statistics; the accuracy of activity range measurement reaches sub-millimeter; 11C is the most dominating positron emission isotope contributing to the overall positron activity; the image quality and the precision of activity range measurement depend on the count statistics, and high count statistics leads to improved image quality and precision. Although the study is preliminary with simple system set-ups, it does provide interesting and important results which should lay the basis leading to future clinically relevant investigations.