The author pointed out that in explaining foaming reaction mechanism of sodium mitrite the previous understanding was inadequate. In this paper, the action mechanism of HNO2 was highlighted. Calculation fomula of gas generation rate and volume was derived. Calculation results shown that [NO2-], [H+] and temperature have influence on generating rate and volume. Since [NO2+] and [H+] can quickly compound, the foaming, reaction rate can not fully control gas generation volume. For effective improvement a special acid was added to generate a catalytic reaction.