Ash-Shaafi'iy ya kafa ilimin Usulul-Fiqh, kuma malamai da masana fiqhu da suka biyo bayansa sun karɓe shi da kyau, ta hanyar bincike da nazari. Don haka suka ƙara masa abubuwa, suka gyara wasu al'amura a cikinsa, har ya zama ilimi mai cikakken tsari, bayyananne, kuma mai bambanta da sauran ilimomi cikin siffofinsa, saboda ginshiƙi mai ƙarfi da Al-Imam Ash-Shaafi'iy ya aza wa wannan ilimi mai daraja da ƙarfi.

Ash-Shaafi’iy founded the ‘Ilm of Usool ul-Fiqh and this was received (well) by the ‘Ulamaa and Fuqahaa, who followed him, through examination and study. So they added to it and revised a number of issues within it until it became an ‘Ilm that was coherent, clear and distinguished in its features, due to the firm foundation that Al-Imaam Ash-Shaafi’iy laid down solidly for this noble knowledge.

Ash-Shafi'iy bai kai ga matakin kammalawa ba dangane da Usulul-Fiqh, don haka sai wasu suka zo domin su kammala ginin da ya fara. Daga cikinsu akwai wadanda suka bi shi suna yin bayani da karin bayani, akwai kuma wadanda suka zo suna kara sabbin ka'idoji, sannan wasu suka zo suna sukarsa da kuma saba masa a wasu daga cikin Qawa'id (ka'idoji).