The adjoint technique is used in the variational data assimilation using the famous Lorenz model. The numerical experiment shows that the four dimensional assimilation has some relation with the model's predictability, with the increase of error of initial guess field, the effect of the assimilation model become even worse, until the assimilation process fails completely. In the time window of assimilation, if the points of observation are enough, the errors of the observation will not affect the effect of assimilation very much, this will be beneficial for the satellite and radar data assimilation used in operational weather prediction model.