The traditional Multi-Attribute Decision-Making （MADM） methods are based on two-way decision， which involve only acceptance or rejection. Three-Way Decision （3WD） reduces decision risks by introducing a deferred decision. In the current complex and dynamic decision-making environments， applying 3WD to provide solutions to MADM problems has emerged as a hot research topic. Therefore， a utility three-way multi-attribute decision-making model based on Pythagorean Fuzzy Numbers （PFNs） similarity was proposed. First， the relative utility function was extended to the Pythagorean fuzzy environment， and a similarity measure of PFNs was integrated， to establish a similarity-based relative utility function. Then， the ideal superior and inferior reference sequences were obtained using the score functions， and grey relational coefficients were computed on the basis of distance measures between PFNs， thus proposing a method for calculating conditional probabilities based on Grey Relational Analysis （GRA）. Based on this， a ranking strategy was introduced via expected utility， thereby obtaining a three-way multi-attribute decision-making model based on PFN similarity. The feasibility of the proposed method was validated through a site selection assessment case for new municipal solid waste treatment plants. Experimental results demonstrate that the proposed method can categorize alternatives precisely and reduce decision risks. Compared to the traditional MADM methods， it incorporates objective weights with 3WD classification to overcome the limitation of only ranking； compared to the three-way MADM methods， its GRA optimized conditional probabilities are more consistent with the classification logic of 3WD.