  Electrons are accelerated at the shock wave diffuse and advect outward, and
subsequently drift away into the emitting region of the jet that is located in
the downstream flow from the plane shock. The current work considers the
acceleration of the electrons in the shock front. Assuming a proper boundary
condition at the interface between the shock and the downstream zones, a novel
particle distribution in the downstream flow is proposed in this work to
reproduce the broadband spectral energy distribution of BL Lac objects. We find
that (1) we can obtain the particle distribution downstream of the shock wave
in four cases; (2) electrons with higher energy ($\gamma>\gamma_{0}$) dominate
the emission spectrum; (3) the distinctly important physical parameters assumed
in our model can reasonably reproduce the multi-wavelength spectrum of the
high-synchrotron-peaked BL Lac object Markarian 421 (Mrk 421).
