A three-dimensional off-lattice protein model with two species of monomers, hydrophobic and hydrophilic, is studied. Enlightened by the law of reciprocity among things in the physical world, a heuristic quasi-physical algorithm for protein structure prediction problem is put forward. First, by elaborately simulating the movement of the smooth elastic balls in the physical world,the algorithm finds low energy configurations for a given monomer chain. An "off-trap" strategy is then proposed to get out of local minima. Experimental results show promising performance. For all chains with lengths 13 ≤ n ≤ 55, the proposed algorithm finds states with lower energy than the putative ground states reported in literatures. Furthermore, for chain lengths n = 21, 34, and 55,the algorithm finds new low energy configurations different from those given in literatures.