Material failure is an important stochastic factor in manufacturing process, based on which, the concept of valid surplus is put forward. The relationship between surplus and valid surplus is discussed. Taking example of the one machine one part type unreliable manufacturing systems where the occurrence of material failure is submitting to Bernoulli distribution,, a controlling thought based on valid surplus is presented. By applying this thought into control of one machine one part type unreliable manufacturing systems, an optimal control model is formulated with valid surplus being the status variable and productivity being the control variable. An optimal control policy is obtained.