The invention relates to a water turbine blade blank profile measuring and machining allowance analyzing method capable of effectively solving the problem of water turbine blade blank profile measurement and machining allowance analysis to ensure the manufacturing quality and precision of the water turbine blades, comprising the following technical solutions: measuring the blade blanks by using adigital close range photogrammetry to obtain the measuring point cloud data shapes of the blank profiles consequently, aligning a measuring coordinate system where the measuring point clouds are located to a design coordinate system where a CAD design model is located, establishing a differential relation of free-form surface parameters with respect to rigid rotation parameters by using an analytic method derived by the multivariate of Newton iteration method to obtain a Jacobian matrix and Hessian matrix which are available for a target equation; then performing the numerical optimization calculation based on the LM method and Newton iteration method, and performing report or graphical output and print on the result. The inventive method is scientific, advanced, stable and reliable, and has high accuracy, thereby ensuring the hydraulic stability, production efficiency and cavitation property, and bringing enormous economic and social benefits.