In the family of polyimide (PI) materials,Upilex-S(R) film has been a shining star through the research PI materials due to its appealing merits.Unfortunately,the wholly rigid-rod backbone and easily formed skin-core micromorphology and microvoids of Upilex-S～ type PI lead to the high difficulty in melt-and wet-spinning fabrication.Herein,we propose a facile and scalable method,reaction-spinning,to fabricate the Upilex-S(R) type PI fiber,in which the rapid solidification of spinning dope and partial imidization take place simultaneously.Thus,the stability and mechanical strength of as-spun fibers can be improved,and the microvoids in fibers can be greatly reduced in relative to the wet-spun fibers.The resultant Upilex-S(r) type PI fiber shows higher tensile strength and modulus than most commercial thermal-oxidative polymeric fibers with an ultrahigh glass transition temperature Tg of 478 ℃.Moreover,the WAXS and SAXS results indicate that orthorhombic crystals are formed for Upilex-S(r) type PI fiber in the post hot-drawing process.Increasing the hot-drawing temperature results in a continuous crystallization and high orientation of PI chains in amorphous phase and perfects the existing lamellar structure,which make a great contribution to the improved mechanical property.