Based on the shell model and random phase approximation theory, and using a shell-model Monte Carlo (SMMC) method, we have investigated the electron capture (EC) for nuclides 56,57,59,60Co, and the rate of change of electron fraction (RCEF) in supernova explosive surroundings. We compared our results, which were obtained by using SMMC method, with those analyzed by using Aufderheide's method. The results show that the EC rates increase greatly, even more than 6 orders of magnitude (e. g. for 57,59,60Co at ρ7=0.43, Ye=0.48). On the other hand, with the increase in temperature and density, the RCEF decreases greatly, even by 5 orders of magnitude (e. g. for 59Co at ρ7=5.86, Ye=0.47). The discussions of error factor show that in a lower density and temperature surrounding, the error is relatively great. But it may be small in the higher density and temperature surroundings.