The morphology of a polymer donor/polymer acceptor active layer in all-polymer solar cells (all-PSCs) has a big influence on photovoltaic performance. It is difficult to tune the active layer morphology of all-PSCs by conventional methods, such as thermal annealing, solvent annealing, and utilization of solvent additive. In this article, we report that the active layer morphology of all-PSCs can be effectively optimized by the temperature of the solution for spin coating. We select poly[4-(5-(4,8-bis(S4(2-butyloctyl)-thio)thiophen-2-yl)-6- methylbenzo[1,2-b:4,S-bldithiophen-2-yOthiophen-2yl)-5,6-difluoro-2- (2-hexyldecyl) -7-(5-methylth iophen-2-yl)-2-H-benzo [d]-[1,2,3] triazole (CD1) as the polymer donor and poly[4-(S-(S-(2decyltetradecyl)-10-(2-dodecylhexadecyl)-4,4,9,9-tetrafluoro-2-methyl-4,5,9,10-tetrahydro-3a,5,8a,10-tetraaza-4,9-diborapyren-7-yl)-thiophen-2-yl)-7-(5-methylthiophen-2-yObenzo[c][1,2,5]thiadiazole] (PBN-12) as the polymer acceptor. Compared with the solution temperature of 30 degrees C, the CD1-PBN-12 active layer spin-coated at the solution temperature of 90 degrees C exhibits smaller phase-separation size and optimal crystallization degree of the polymers. The resulting all-PSCs device shows improved charge separation, balanced charge transporting, and suppressed bimolecular recombination. As a result, the power conversion efficiency is enhanced from 7.7% to 10.0%. The effect of solution temperature on phase-separation morphology of all-PSCs can be explained by the increased diffusion rate of polymer chains in the film-forming process and the disaggregation of polymer chains in solution at high temperature. These results indicate a new approach for improving active layer morphology of all-PSCs by solution temperature.