The invention provides a thermal treatment process capable of reducing the microsegregation of directionally-solidified high-niobium TiAl alloy. The high-niobium TiAl alloy containing trace elements such as W, B, Y, Mn and the like is prepared from the following components by atomic percent: 44-46% of Ti, 6-9% of Al, 0-2% of Nb, 0-0.5% of W and Mn, and the balance of B and Y; and a fastener ingot is obtained by virtue of electric arc melting, the fastener ingot is sucked and cast into an alloy test bar, and a directionally-solidified high-niobium TiAl alloy sample is prepared by utilizing a Bridgeman method. A directional solidification technology is utilized, so that alloy grains grow along a forced direction, the transversal grain boundary is eliminated, and the high-temperature property is improved, but as the cast high-niobium TiAl alloy has structural defects caused by ubiquitous S segregation, beta segregation and alpha segregation, the mechanical property of the alloy is finally influenced. According to the invention, heat preservation is carried out on the directionally-solidified sample at the temperature of 1250-1290 DEG C for 6-24 hours, and when the sample is cooled to 890-910 DEG C along with a furnace, heat preservation is carried out for 30-35 minutes, thus the beta segregation and the alpha segregation are greatly reduced, simultaneously recrystallization and grain growth phenomena do not appear, and the mechanical property of the alloy is effectively improved. The process provided by the invention is simple and reliable in method, low in cost and strong in practicability; and the obtained high-strength, high-toughness and directionally-solidified high-niobium TiAl alloy has a wide application prospect in the aerospace field.