In the cosmic radiation environment , a large number of cosmic rays often lead to the transient faults in onboard computers ,which result in data errors or control flow errors during program execution. Against program execution errors caused by transient faults, a softwareimplemented fault detection algorithm named SITFT is proposed, which not only detects data errors but also detects control flow errors effectively based on program redundancy and signature analysis. The results of fault injection experiment show that the misdata in the SITFT program output decrease by 49.0%－73.2% on the precondition that the execution time increases by 58%－111% and the memory cost increases by 153%－225% than the original program execution.