The number of cold atoms trapped in magneto-optical trap(MOT) can be derived from probe absorption signal. After the MOT is switched off, the cold cloud freely expands in gravitational field with an initial velocity distribution. So the measurement process can be described with the interaction model between the individual stationary two-level atom and single mode resonate laser. Absorption method can largely improved the measurement precision. In this paper we introduce the physical principle of measurement and demonstrate the whole experimental procedure in detail. The number of atoms trapped in our MOT, (8±0.3)×106 is measured in experiment. And the number of steady-state atoms and the lifetime of atom trapped in MOT was used to deduce the capture rate of MOT.