The paper aims at analyzing the basic thought and connotation of seeking truth from the facts and the application and development of seeking truth from the facts in socialism with Chinese characteristics, so as to provide theoretical basis for the implementation of the thought of seeking truth from the facts. Firstly, the theoretical basis of Marxism was studied deeply, and it was discussed from practice theory, historical materialism, and dialectical materialism. Then, Marxist philosophy theory was used to discuss the basic thought and connotation of seeking truth from the facts under socialism with Chinese characteristics. The results showed that, under the socialism with Chinese characteristics, the basic line of seeking truth from the facts is "starting from reality, integrating theory with practice, seeking truth from the facts, and testing and developing truth in practice". The theory of seeking truth from the facts is the basic core of the development of the party and socialism. The basic premise of seeking truth from the facts is that everything starts from reality, and the essence is to use materialism to explain and answer philosophical questions. To sum up, the theory of seeking truth from the facts in Marxist philosophy is the basic theory for the development and construction of socialism with characteristics in China, and the core idea of the Communist Party of China. On the road of developing socialism with characteristics, it is supposed to adhere to the combination of theory with practice, start from the root, and get the test truth from practice.