The effect of plasma density scale length on the conversion efficiency of high order harmonic generation in the interaction of intense laser and plasma was investigated via PIC (particle in cell) simulation. The conversion efficiency of high order harmonic generation produced by the interaction between p polarized, non relativistic laser pulse and over dense plasma with different density scale lengths was calculated. The plasma density scale length significantly affects the conversion efficiency and this effect is different for different harmonic order, different laser pulse duration and different plasma density.