The invention employs industrial raw material of mixed xylene (xylene for short) as raw material to generate, via oxidation, a mixture of mixed benzene dicarboxylic acid (MA) and phenylacetic acid, and then phenylacetic acid products can be obtained by separation and purification. The mixed phthalic acid (MA) (a mixture of terephthalic acid, m-phthalic acid and phthalic acid) is refined to obtain high-purity mixed phthalic acid (PMA) products. The pure mixed phthalic acid (PMA) products, skipping the production procedures of para-xylene (PX), is a novel raw material of polyester resin, which can generate poly-mixed polyethylene terephthalate (PEM) via esterification and condensation polymerization as an alternative of pure terephthalic acid (PTA) with ethylene glycol (EG). Compared with general polyethylene terephthalate (PET), the poly-mixed polyethylene terephthalate (PEM) has the characteristic of low melting point, and is suitable for being used as a plastic raw material and low in production cost. The pure mixed phthalic acid (PMA) can be mixed with the pure terephthalic acid (PTA) in accordance with random proportion and is used for preparing various differentiated polyester resins, such as bottle grade polyester resin, film grade polyester resin and the like.