Effects of external stress applied to polarizationmaintaining(PM) fiber pigtail of a Ybranch multifunctional integrated optical device on extinction ratio as well as its temperature stability are researched.Based on the analysis of stress on the fiber,expression of extinction ratio as a function of the magnitude and direction of the stress is deduced,and the temperature stability of extinction ratio is discussed.Results show that the external stress is the main factor which affects extinction ratio and its stability.When the direction of stress is at an angle from the birefringent axes of a free PM fiber,the maximum extinction ratio achieves during fiberchip coupling is smaller than that of a free fiber.As a result,the polarizing direction of the chip and the slow axis of the fiber will be misaligned.The change of the stress in magnitude and direction caused by temperature results in the fluctuation of the extinction ratio.While the stress is along the birefringent axes of the fiber,it has no effect on extinction ratio,and the polarizing direction of the chip and the slow axis of the fiber can be precisely aligned.Fiber pigtails fixed with a Vgroove and with a Ugroove are analyzed in comparison,and test results show that a fixing block with a Vgroove can improve extinction ratio and its stability effectively.