Recently,12442 system of Fe-based superconductors has attracted considerable attention owing to its unique double-FeAs-layer structure.A steep increase in the in-plane upper critical field with cooling has been observed near the superconducting transition temperature,Tc,in KCa2Fe4As4F2 single crystals.Herein,we report a high-field investigation on upper critical field of this material over a wide temperature range,and both out-of-plane (H||c,Hcc2) and in-plane (H||ab,Habc2) directions have been measured.A sublinear temperature-dependent behavior is observed for the out-of-plane Hcc2,whereas strong convex curvature with cooling is observed for the in-plane Habc2.Such behaviors could not be described by the conventional Werthamer-Helfand-Hohenberg (WHH) model.The data analysis based on the WHH model by considering the spin aspects reveals a large Maki parameter α =9,indicating that the in-plane upper critical field is affected by a very strong Pauli paramagnetic effect.