At present, more attention has been paid to the performance and lightweight design for passenger car seats. The optimisation design for seat frame is performed in this research. The innovation is that a particular method of lightweight optimisation is proposed according to the characteristics of seat frame. The best thickness-material scheme of the parts that needs to be optimised (opti-parts) is obtained by applying the method of grey relational analysis (GRA), grey entropy measurement, and average strain indices (GRA & GEM(ASI)). Moreover, in order to verify the advantages of GRA & GEM(ASI), a comparison is made among several different techniques. The results show that, not only the gross cost and gross mass of opti-parts are reduced by yen 3.13 (16.68%) and 0.67kg (16.88%), respectively, but also the performance and reliability of seat frame is well guaranteed. The method of GRA & GEM(ASI) proposed makes significant contributions effectively performed in multi-objective optimisation design for passenger car seat frames.