The article focuses on analyzing the secondary teacher school training in the South of Vietnam under the Saigon regimes (1955-1975) in the two aspects: training in pedagogical universities and retraining in professional activities. This is an important aspect to study secondary school teachers in particular and teachers in general - a part of the Southern intelligentsia. The article consists of the three main contents. Based on the original documents stored at the National Archives Center II and the statistics and memoirs from interviews with historical witnesses, the article outlines the current situation of the secondary school teacher training and their professional improvement in the South of Vietnam from training objectives, processes, programs, and activities to training results. Secondly, the article points out the advantages and disadvantages of the secondary school teacher training. The outstanding advantage is the strict training process and preferential treatment. As a result, the secondary school teacher gains an important position in the classroom and is highly respected by society. The secondary school teacher also has many disadvantages. Typically, private secondary school teachers are almost untrained and not improved their professional knowledge. These disadvantages stem from inequalities between public and private secondary schools and policy constraints on secondary school teachers. Finally, the study evaluates the impact of the secondary school teacher training and their professional improvement on the movement of the secondary school system and the development of education in the South of Vietnam during this period.