The purpose of this research is to examine the language policies of Laos in different historical periods: the Kingdom of Lan Xang and the vassal state of Siam, the French colonial period, the monarchical period, and the socialist period. Based on the three-dimensional model of language policy research proposed by Lo Bianco and Aliani, "Text", "Discourse", and "Practice", through analyzing the Lao policy documents that state the position of language use in different periods, unscrambling the literature on the interpretation of policy documents, and investigating the actual use of language, the following conclusions are drawn: during the period of Kingdom of Lan Xang and the vassal state of Siam, the original Lao was similar to the Yue language spoken by the Baiyue people in the history of southern China. The Lao script is alphabetic writing created from Sanskrit and Pali. More than a century of Siamese rule over Laos resulted in a high degree of similarity between the Lao and Thai languages today. During the French colonial period, the Language policy of assimilation, standardization, and “de-Siamization” was implemented in Laos. In the meantime, French had been the official language, working language, and instruction language, while Lao was an optional subject in the education system. During the monarchical period, it was a transitional period in terms of language policy in the Kingdom of Laos. The Royalist Government of Laos continued the language education policies and spelling rules of Lao formulated by the “Comité Littéraire” of French colonial authorities and established Lao as the official language in the Constitution. During the socialist period, the government of the Lao People’s Democratic Republic adopted the language policy of monolingualism by taking Lao as the official language, national language, and instruction language. The monolingual policy aroused opposition from other ethnic minorities, but it played a role in cultivating Lao national consciousness and constructing a national identity for an emerging multi-ethnic country. While fulfilling this historic mission, the Lao government has gradually improved the monolingual policy, adjusted the policy of minority languages, and implemented the policy of the coexistence of multiple languages.