As an important branch of biomedical information extraction, Protein-Protein Interaction extraction (PPIe) from biomedical literatures has been widely researched, and machine learning methods have achieved great success for this task. However, the word feature generally adopted in the existing methods suffers badly from vocabulary gap and data sparseness, weakening the classification performance. In this paper, the unsupervised word representation approach is introduced to address these problems. Three word representation methods are adopted to improve the performance of PPIe: distributed representation, vector clustering and Brown clusters representation. Experimental results show that our method outperforms the state-of-the-art methods on five publicly available corpora.