I want to run stoch_simul after MCMC and I want to set the parameters of the model to their mean value in the posterior distribution.
What shoud I do after stimation? how I should use Bayesian_irf ?

bayesian_irf will generate mean IRFs. As you want the IRF at the mean, you can simply call stoch_simul after estimation. Dynare will set the parameters to the posterior mean after estimation and running stoch_simul will then generate the IRFs at these parameter values.