Ultrathin mixed matrix membranes (MMMs) containing two-dimensional (2D) fillers are highly sought recently due to their good performance in gas separations. Here we report the preparation of PIM-1 based ultrathin MMMs containing 2D metal-organic framework (MOF) nanosheets using spin coating method. The lamellar structure of the 2D layered MOF filler (copper 1,4-benzenedicarboxylate nanosheets, CuBDC-ns) enlarges the contacting area between fillers and the polymer matrix, leading to the formation of dense MMMs. The centrifugal force generated during the spin coating process helps to horizontally align these CuBDC-ns, increasing the gas diffusion resistance across the membrane. The influence of membrane thickness and filler content on the separation performance of CO2/CH4 mixture is examined and discussed. Optimizing membrane thickness and filler content results in a 660 nm thick ultrathin MMM comprising 10 wt% CuBDC-ns, which exhibits a CO2/CH4 selectivity of 15.6 and CO2 permeance as high as 407 GPU, surpassing other PIM-1 based membranes. Besides, the long term stability test shows that the prepared ultrathin MMMs are stable up to 100 h. These results demonstrate that the ultrathin MMMs reported in this study are excellent candidates for CO2/CH4 separation, with potential applications in natural gas purification and biogas upgrading.