Self-sensitization theory of expanded ammonium nitrate (AN) is the key point of the present innovation .On the basis of heat points initiating mechanism of explosion, micro air bubble is embedded in explosive of expanded AN. the theoretical basis of self-sensitization is provided through calculation of critical heat point temperature of micro air bubble. Comparing expanded AN with common AN and pearlstone, the micro air bubble distribution and its regularity in expanded AN are given which provide powerful basis assurance for its success of self-sensitization.