We present a scheme for microwave photon detection via cross Kerr nonlinearity between two microwave modes mediated by a transmon. Using a cascaded, stochastic master equation method and conditional measurement simulations, we analyze the dependence of the probe phase shift on the control photon number. We demonstrate here that for a microwave pulse with a few photons (below 10), the relationship between the phase shift and the photon number is almost linear. Also, in our scheme, microwave photons to be detected are guided by a 1D transmission line, rather than a cavity, making it suitable for a wide range of frequencies. Finally, the signal-noise-ratio (SNR) and the feasibility of this scheme for experimental realization are discussed. Our scheme might help to build a practical photon detector in the microwave regime.