This study proposes an equivalent crack life prediction method for root cracks in T‐welded joints. An equivalent initial defect is used to describe the severity of the weld defect. Fatigue life prediction is achieved based on equivalent fracture parameters and high‐stress fatigue life reference points fitted to obtain a P‐S‐N curve for the entire stress range. Taking the 6082‐aluminum alloy T‐welded joints as the object, the test data under tensile load P‐S‐N curve fitting and crack growth simulation were carried out to compare and verify the applicability under constant and equivalent spectral load. This method enables deterministic fatigue life prediction with less testing and cost. Meanwhile, the approach incorporates the uncertainty of the physical model of the weld into the P‐S‐N curve, simplifying the process of calculating the probabilistic fatigue life. Highlights An equivalent crack life prediction method is developed to study T‐welded joints root cracks. The suitability of using a set of high stress fatigue life reference points to predict P‐S‐N curves is discussed. The applicability of the method for constant and equivalent spectral loads is verified.