With an analytical framework comprised of variables such as parliamentary electoral system, political party system in the parliament, constitutional system (semi-presidentialism) and government type, this paper explores the differences among the constitutional operations from different combinations of the semi-presidentialism subtype (premier presidentialism and president parliamentarism) and the parliamentary electoral system (plurality with single-member-district system, proportional representation system, mixed-member proportional system and mixed-member majoritarian system) in the democracies all over the world. As far as semi-presidentialism is concerned, it is found that the overall constitutional operation under premier presidentialism, no matter which parliamentary electoral system is adopted, goes better than that under president parliamentarism. This finding can be another supportive reference for the perspective which believes premier presidentialism is better than president parliamentarism. Furthermore, the constitutional operation under the combination of president parliamentarism and plurality with a single-member-district system (or mixed-member majoritarian system) is sometimes proceeds more smoothly than that under the combination of president parliamentarism and proportional representation system (or mixed-member proportional system), but sometimes it does not, and even becomes more obstructed. This shows the dilemma of how to choose a suitable parliamentary electoral system under president parliamentarism. Besides, this paper also observes the overall tendency of semi-presidential democracies’ choices for the presidential electoral system, and points out two common misconceptions. The first is the belief that whether the president has great constitutional powers or not is related to the presidential electoral system, and the second is the belief that the president elected via a plurality system has less democratic legitimacy than the president elected via a majority system. These two perspectives do not correspond to actual experiences, and should be clarified.