Abstract: Methods based on extension rule are new approaches for automated theorem proving and can efficiently solve problems with high complementary factor. In this paper, a new strategy to re-implement ER, which is an algorithm based on the propositional extension rule, is proposed. The new implementation of ER is superior to the original one. Based on this, the extension rule is applied in the following three areas: Firstly, there exist a set of analogous SAT problems being solved in real applications. In contrast with solving these SAT problems separately, an algorithm called nER that solves them as a whole is developed. The algorithm nER exploits the repetition property of ER and generally costs less time than the total time of using ER to solve every problem. Furthermore, based on ER, two new algorithms called #ER and #CDE are proposed, the latter being a combination of #ER and #DPLL. Experimental results show that #ER outperforms #DPLL on a wide range of problems and the #CDE integrates advantages of #ER and #DPLL. Finally, an ER based SAT solver is embedded into the conformant fast-forward to study the potential of ER based methods in artificial intelligence planning. Preliminary results show the efficiency of ER and future research topics.