This paper proposes a validation approach, based on simulation, which addresses problems related to both state space explosion of formal methods and low coverage of informal methods. Formal methods, in particular model checking, are used to aid the simulation process in certain situations in order to boost coverage. The invocation frequency of the model checker is dynamically controlled by estimating certain parameters, based on statistics collected previously during the same validation session, in order to minimise verification time and at the same time achieve reasonable coverage. The approach has been demonstrated feasible by numerous experimental results.