ABSTRAK Contemporary jurists view maqashid al-syari'ah as the most rational and relevant fiqh epistemology. This concept has previously been discussed by al-Ghazali in the book al-Mustasyfa. Al-Ghazali made an argument for the validity of the maqashid al-syari'ah based ushul method as legal reasoning. Therefore, this research is focused on the maqasid al-syari'ah based ushul method in al-Ghazali's monomental work, and the argumentation of the validity of the maqhasid al-syari'ah based ushul method as legal reasoning. To answer this question, a content analysis method is used. The first novelty, in al-Mustasyfa al-Ghazali, puts istishlahbased on maqashid al-syari'ah as legal reasoning that is both flexible and strict. Second, istishlah based on maqashid is valid as a proof of legal reasoning even if it is not indicated by certain texts (nash) on condition that it is in line withsyara' actions and does not conflict with texts (nash) or ijma', has qath'i or dhann status that is close to it, is at the daruriyah or hajiyah level which has the same position with daruriyah, but in certain cases must be qath'i, dharuri and kulli. The first conclusion, the legal reasoning based on the istishlah method based on maqashid al-syari'ah with its provisions, shows that al-Ghazali's fiqh methodology is consistent with the basis of Shafi'i madhab ushul, even though it accepts the principle of mashlahah al-mursalah. Second, the reconstruction of al-Ghazali's fiqh methodology has implications for changing the conception of textual fiqh that dwells on linguistic and qiyas boundaries into contextual conceptions with the urgency of the spirit of syara' in legal reasoning. Keywords: Legal Reasoning, Maqashid al-Syari'ah, Karya Monomental al-Ghazali