The invention relates to a method of determining a fault number of rail vehicles in a reliability growth period according to confidence. According to the method, running mileage or time of a project rail vehicle is combined to calculate an expected fault number of the reliability growth period according to reliability index requirements which are specified by a project rail vehicle specification and of a start point and an end point of the reliability growth period, and the allowed project fault number of the reliability growth period is determined according to the confidence to serve as a basis of verifying reliability meeting. By adopting the method, impacts due to rail vehicle batch and number differences of the reliability growth period in gradual delivering of the same-project rail vehicles can also be quantitatively calculated. Traditional methods do not consider randomness of fault occurrence and the impacts of the sample number difference of the rail vehicle number on the allowed reliability fault number, and do not conform to a law of data statistics; but adopting the confidence accepted by both parties and considering the project rail vehicle sample number difference to determine the allowed fault number is a reasonable and feasible method.