At present, under the new situation of societal transformation, economic transition and transportation reform, projects 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 for choosing universities, graduate enrollment units currently face the challenges that how to accurately evaluate postgraduate source quality and attract more high-quality postgraduates. In this paper, a diversified postgraduate source quality evaluation system oriented to professional ability was established firstly according to the requirement of developing China’s transportation strength for first-class talents. Secondly, random forest algorithm was 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 was proposed based on relative characteristic importance of attraction indicators. Finally, taking the discipline of bridge and tunnel engineering in Harbin Institute of Technology for example, the actual effectiveness of postgraduate source quality improvement reform in the past five years was discussed from the perspective of postgraduate development path, which could provide valuable reference of postgraduate enrollment work for other traditional transportation universities.