Time-lens is an analogy of the conventional lens in the time-domain, which constitutes a source of inspiration to design and implement optical systems based on concepts borrowed from the well-established field of Fourier optics. We propose a design method of chirped pulse amplification systems based on this analogy. In this simple physics picture, the relation between dispersion and nonlinear phase shift under gain saturation can be obtained in a straightforward way. We demonstrate that this simple model agrees well with experimental results. It is a simple, effective and practical design method for chirped pulse amplification systems, especially for ultrafast fiber amplification systems.