The electoral system in Hong Kong had experienced significant changes in past decades. The electoral system of geographical constituencies in LegCo had been changed from "double-seat, double-vote" to "single-seat, single-vote", and then proportional representation. The purpose of this study is to analyze the political effect of electoral reform, especially its impact on the party system in Hong Kong. By quantitative analysis of electoral results, it is found that there are effects according to classical theories, as well as different characteristics. The reform of electoral system had led to fragmentation, weakening, and radicalization of party system, and confined political development in Hong Kong.