The invention discloses an AADL based embedded software reliability model building and assessment method. The method comprises the steps that 1, an AADL reliability model of an embedded software is built; 2, the reliability elements of the AADL reliability model are extracted, a reliability model ZAL is built using a Z language, the model comprises a fault model, a structure model and a behavior model; 3, the discrete time Markov chain DTMC is used to describe the ZAL model, and a PCTL formula of describing the embedded software reliability is provided, the PCTL formula is computed using the proposed reliability assessment algorithm ZARE to perform the calculations, instantaneous availability is obtained, and reliability assessment is completed. The reliability model building and the assessment method expresses the embedded software in layers, clearly and concisely and is easy to understand, and ZAL as a formalized model can use model checking and similar methods to conduct rigorous analyses and assessments for reliability.