The maechanism of loading and collision loss in magneto-optical trap has discussed. The Doppler forces and population distribution in excited and ground states were calculated using 1-dimensional multi-level system including σ+－σ－trapping and repumping lights. To avoid collision loss and get effective damping force we discussed the optimal intensity and detuning ranges of repumping light. The experimental loading curves were derived. The maechanism of loading and collision loss in magneto-optical trap has discussed. The Doppler forces and population distribution in excited and ground states were calculated using 1-dimensional multi-level system including σ + -σ-trapping and repumping lights. loss and get effective damping force we discussed the optimal intensity and detuning ranges of repumping light. The experimental loading curves were derived.