A design scheme of flexible cold strap according to the requirement of long linear IRFPA packaging was presented in this paper, the characteristics of long linear IRFPA and linear pulse tube cooler were summarized, design ideas and design methods based on the mechanical and thermal requirements of cold strap were proposed. For typical butting substrate size and working temperature, used partial flexible cold strap scheme was used, which was optimized by FEM tool, design requirements was achieved. Test dewar was made according to the design result, temperature difference of cold strap, flatness and temperature uniformity of butting substrate were verified. The test dewar has passed vibration testing. The test results show that the design scheme is correct and feasible.