In this paper, PZT piezoelectric ceramic specimens with 4 compositions (Zr/Ti=50/50, 53/47, 56/44, 58/42) in $Pb(Zr,Ti)O_3$ system were fabricated. We studied effects of poling strength and thermal aging on the temperature characteristics of eletromechanical coupling factor k31 of the specimens, which were poled with the DC electric fields, 1.5, 2.5 and 3.5 kV/mm respectively and thermally aged for an hour at $200^{circ}C$. The eletromechanical coupling factor k31 of the specimen with the composition Zr/Ti= 53/47, nearest to the morphotropic phase boundary decreased the most greatly, irrelevant to the intensity of poling field, due to 1st thermal aging. And the temperature coefficient of eletromechanical coupling factor k31 was (-) in the tetragonal phase composition and (+) in the rhombohedral phase composition, which is reverse in the temperature coefficient of resonance frequency. It is interesting that eletromechanical coupling factor k31 of PZT ceramics is shown to be able to increase as temperature increase in the interval $-20{sim}80^{circ}C$.