It is necessary to analyze the concentration ratio for the design of concentrating solar thermal power system. Linear Fresnel reflector (LFR) can be treated as a smooth linear optical reflector that is broken into segments. Every mirror segment or row of this kind of concentrator needs to track the sun in real time and reflects the sunlight to a fixed linear receiver. Therefore, the bandwidth on the absorber illuminated by every mirror row varies continuously, which makes the analysis of concentration ratio very complex. Firstly, the formulae for the projected angles of incident and reflected solar rays, the tracking inclination angle and the bandwidth illuminated on the flat plane absorber are obtained by a two-dimensional optic analysis. Secondly, the expressions of geometric concentration ratio of LFR are derived and the dependence of ideal geometric concentration ratio on number of mirror slats and relative distance is analyzed. And then, optimization of mirror field width and tower height is analyzed. Finally, the effect of angular size of the sun′s disc on geometric concentration ratio is illuminated.