Abstract: The foundation reinforcement or reconstruction of existing buildings under soft soil conditions is faced with many complicated conditions. Construction safety and surrounding environmental protection are particularly important. A small diameter steel pipe-rotary jet composite pile process is proposed. Through field test research on various bearing capacity of small diameter steel pipe-rotary jet composite piles, strain data of steel pipe pile are abtained, and its bearing characteristics and force transmission mechanism are analyzed, summarizing and proposing the design calculation formula of small diameter steel pipe-rotary jet composite pile are summerized and proposed, which provide a theoretical basis for engineering applications.