The flux density distribution on the plane photovoltaic cells of parabolic trough concentrator is theoretically studied. The results show that the flux density on plane photovoltaic cells distributed in three regions, which from the center to both sides are flux density is zero, flux density is greater than incident light and flux density is smaller than incident light. The flux density distribution curve is fitted and flux density goes down exponentially from the center to the edge. The closer the photovoltaic cell stay to parabolic focal and higher flux density is obtained and more quickly the flux density goes down. The validity of the result is demonstrated by comparing result with that in references.