Sophisticated efficient electrocatalysts are essential to rectifying the shuttle effect and realizing the high performance of flexible lithium-sulfur batteries (LSBs). Phase transformation of MoSe 2 from the 2H phase to the 1T phase has been proven to be a significant method to improve the catalytic activity. However, precisely controllable phase engineering of MoSe 2 has rarely been reported. Herein, by in situ Li ions intercalation in MoSe 2 , a precisely controllable phase evolution from 2H-MoSe 2 to 1T-MoSe 2 was realized. More importantly, the definite functional relationship between cut-off voltage and phase structure was first identified for phase engineering through in situ observation and modulation methods. The sulfur host (CNFs/1T-MoSe 2 ) presents high charge density, strong polysulfides adsorption, and catalytic kinetics. Moreover, Li-S cells based on it display capacity retention of 875.3 mAh g −1 after 500 cycles at 1 C and an areal capacity of 8.71 mAh cm −2 even at a high sulfur loading of 8.47 mg cm −2 . Furthermore, the flexible pouch cell exhibiting decent performance will endow a promising potential in the wearable energy storage field. This study proposes an effective strategy to precisely control the phase structure of MoSe 2 , which may provide the reference to fabricate the highly efficient electrocatalysts for LSBs and other energy systems.