Abstract: Through the description of straightforward shooting of a straight running torpedo, the random error and some concepts of hitting problem are studied, and the difference between success probabilities based on true value and ob-served value of target movement element is pointed out. For objective probability, two analytic methods of computing probability are proposed based on the first order or second order approximation of the function of random error vector. According to a theorem about the formulas to compute the mean value, variance and eigenfunction of a common second order function of normal distributed random vector, an analytic method with the second order approximation is given. Results show that the two analytic methods have high precision, and the first order approximation method is better than the second order one in the case of computing the objective hitting probability of torpedo straightforward shooting.