Recent simulations of the solid phase DNA amplification (SPA) by J.-F. Mercier et al (Biophys. J. 85 (2003) 2075) are generalized to include two kinds of primers and the off-lattice character of the primer distribution on the surface. The sigmoidal character of the primer occupation by DNA, observed experimentally, is reproduced in the simulation. We discuss an influence of two parameters on the efficience of the amplification process: the initial density p_0 of the occupied primers from the interfacial amplification and the ratio r of the molecule length to the average distance between primers. The number of cycles till the saturation decreases with p_0 roughly as p_0^{-0.26}. For r=1.5, the number of occupied primers is reduced by a factor two, when compared to the case of longer molecules. Below r=1.4, the effectivity of SPA is reduced by a factor 100.