Based on the data of Chinese listing corporations from 2012 to 2014, their levels of related and unrelated diversification are calculated with the application of entropy method. The effects of different types and levels of diversification on research and development (R&D) investment intensity are examined by path analysis model, as well as the mediating roles of organizational slack. The results reveal that, related diversification increases R&D intensity by accumulating organizational slack, whereas unrelated diversification decreases R&D intensity by dissipating organizational slack. At last, several recommendations are proposed from the aspects of exploiting organizational slack and adjusting diversification scope.