Due to its advantages, such as shock resistance, low power consumption, and so on, solid state drives are considered as a next generation storage device. And write amplification is a very import system parameter for evaluating performance. To simplify the analytical model of write amplification for solid state drives adopting page-level address translation mechanism, this paper proposes a probability model. Furthermore, numerical results based on our proposed probability model are also obtained. This paper can pose as a guideline for a first-order estimation of the write amplification for parameter ranges in solid state drives taking page mapping scheme.