In order to calculate exactly mean damage probability of first-round fire of dispersive projectile of canon launch to assembly targets, a calculation model of damage probability of first-round fire is built up for dispersive projectile in case of the deviation of a target from aiming point being (ax, az). According to method of “turning for quadrants integration into one quadrant integration”, calculation formulas of damage probability of first-round fire are deduced on condition of the deviation of a target from aiming point being (ax, az), and the condition which damage probability met is analyzed. The problem to calculate mean damage probability of first-round fire of dispersive projectile to assembly targets is discussed, and the method of the calculation is given. Result of the example shows that mean damage probability of first-round fire of dispersive projectile to assembly target can be calculated effectively by the given method.