We describe a direct model for simulation of relativistic electrons acceleration with a given electromagnetic field which is determined by wave packet parameters. The multimode time-spatial structure of a focused Nd-laser beam with stochastic phase disturbances of each spectral component is taken into account as a source of random forces. Electron energies of more than 10 MeV are obtained even at moderate flux densities of 1016 W/cm2. The developed numerical code makes it possible to obtain a quantitative energy distribution function in relation to both field intensity and the temporal bell-shape of the laser pulse. The efficient heating of electrons can be triggered in the presence of a counter propagating wave being reflected from the critical plasma area with a different reflection coefficient. The heating mechanism occurs with a delay relative to the beginning of the pulse when the laser fields exceed some threshold amplitudes. The qualitative comparison of simulation results with the experimental data is given as evidence that this mechanism is reasonable.