paa solution at a molar ratio bpda oda ppda bia 6 1 3 2 three diamine monomers oda p pda and bia are put in a three necked flask at first then measured solvent dmac is added oda p pda and bia are mechanically stirred at 50 u00b0 c under the protection of nitrogen and are fully dissolved then bpda is added in batches and stirred to ensure its solid content is 25 then under the protection of nitrogen it is stirred 10 h at 50 u00b0 c and 5 h at u2212 10 u00b0 c to obtain a viscous paa solution with intrinsic viscosity of 3 1 dl g preparation of pi fiber after the paa solution is filtered and defoamed it is spun by a wet process after the spinning solution is pumped out by a metering pump it passes the spinneret plate number of bores 500 bore diameter 0 55 mm and enters a coagulating bath it comprises water and dmac with a volume ratio of 1 1 to form paa fiber after it is washed in a washing bath comprising water and ethanol with a volume ratio of 3 1 it directly enters the four stage heat furnaces in turn the temperature of the heat furnaces is 160 u00b0 c 270 u00b0 c 350 u00b0 c and 500 u00b0 c respectively and the draw ratio is 2 1 5 1 1 and 5 5 respectively in the end the yarn is wound into a.