The rules of fiqh (qawā'id al-fiqhiyyah) have a goal, namely as the determination of Islamic law from various new problems due to progress and changing times, including in the economic field (muamalah maliyah) in Islamic financial institutions. By making the rules of fiqh as the legal basis, it is a must to get convenience from contemporary economic problems which do not have the dominant texts of shar'ih in the Qur'an and hadith. Even in financing the results of Islamic banking through musharaka. With the public's interest in this financing, it is necessary to increase understanding related to the rules of fiqh in Islamic law. So the purpose of this study is to explain or describe the rules of fiqh as a legal principle in musharaka financing transactions. The method used is qualitative. Data collection techniques in this study were sourced from primary and secondary data. Primary data comes from the Qur'an, hadith, and fatwas in the DSnMUI, while secondary data comes from books, journals, and research reports related to the implementation of qawā'id al-fiqhiyyah.