The article presents the process of deforming parliamentary democracy in Bulgaria after the First World War. The analysis begins with the presentation of fundamental political principles and relations between the main state institutions in the Tyrnovo Constitution, the fundamental law of Bulgaria in this period. Further, the problems of Bulgarian parliamentarism are discussed. Parliamentary democracy in Bulgaria in the interwar period was deformed by violating the rules of the parliament and subordinating it to party interests. It was further weakened by the usurpation of power (two coups d’état), disrespect for the constitutional principles, and by regulations prohibiting the activity of certain political forces. As a result, Bulgaria has evolved from a parliamentary monarchy that respecting the parliamentary system and procedures to the dictatorship.