Electrical steels (ES) are the core for electrical motors, power generators, etc. To restore the electromagnetic property of ES after stamping, stress-relieving annealing is required. However, annealing can be very destructive to the insulation coating on ES and an inappropriate annealing usually results in coating adhesion failure, which badly affects the performance of ES facilities. Therefore, effects of the atmosphere in a practical (not absolutely air-tight) furnace on the post-annealing adhesion capability of ES coating were investigated to qualitatively reveal the influences of O2, CO2, H2, and H2O, respectively. The presence of CO2 has little effect while both O2 and H2O can deteriorate the post-annealing adhesion capability. Contrarily, H2 has protecting effects so that good post-annealing adhesion capability of coating can be fulfilled. Therefore, to ensure good post-annealing adhesion capability to the coating, minimal contents of oxygen and humidity in the furnace atmosphere is required and the presence of some hydrogen in the furnace atmosphere can be helpful.