With a simple evolution model of the magnetic fields of isolated radio pulsars, the statistical evidence for them is investigated, Monte-Carlo techniques are used to generate sample pulsars of particular distributions of initial periods. period derivatives, magnetic fields. luminosities. and Galactic positions. The period-dependent beaming fraction of pulsars is considered in the simulation, Then the best fits by comparing the synthesized populations with the observed pulsars are made, It is shown that the field decay model may also have some consistency with the current understanding of pulsar statistics.