At present, under the new situation of societal transformation, economic transition and transportation reform, initiatives for developing China’s transportation strength urgently need a large number of top-notch innovative talents with morality, ability and global vision. However, with continued expansion of postgraduate enrollment and constant change of candidates’ motivation in choosing universities, universities currently face the challenges of how to accurately evaluate postgraduate source quality and attract more high-quality postgraduates. In this paper, a diversified postgraduate source quality evaluation system oriented towards professional ability is established firstly according to the requirement of developing China’s transportation strength for first-class talents. Secondly, random forest algorithm is used to deeply explore the key factors affecting candidates’ motivation for choosing universities considering hard power in education and scientific research, soft power in human resources and future development potential. Thirdly, a multi-measure reform proposal for improving postgraduate source quality in the discipline of bridge and tunnel engineering is proposed based on relative characteristic importance of attraction indicators. Finally, taking the discipline of bridge and tunnel engineering in Harbin Institute of Technology as an example, the actual effectiveness of postgraduate source quality improvement reform in the past five years is discussed from the perspective of postgraduate development path, which could provide valuable reference to postgraduate enrollment work for traditional transportation universities.