Many economically important crops and vegetables belonging to the cruciferous family are heavily endangered by clubroot disease caused by Plasmodiophora brassicae infection. Breeding of clubroot resistant cultivars based on mapping and cloning of resistant genes is commonly regarded as the most cost-effective and efficient way to fight against this disease. The traditional way of R gene functional validation requires stable transformation that is both time- and labor-consuming. In this study, a rapid and efficient hairy-root transgenic protocol mediated by Agrobacterium rhizogenes was developed. The transformation positive rate was over 80% in Brassica napus showed by GUS reporter gene and this transformation only took 1/6 of the time compared with stable transformation. The system was applicable to different B. napus varieties and other cruciferous crops including Brassica rapa and Brassica oleracea. In particular, two known CR genes, CRA3.7.1 and CRA8.2.4 were used respectively, as example to show that the system works well for CR gene study combined with subsequent P. brassicae infection in B. napus. Most importantly, it works both in over-expression that led to disease resistance, as well as in RNAi which led to disease susceptible phenotype. Therefore, this system can be used in batch-wise identification of CR genes, and also offered the possibility of manipulating key genes within the P. brassicae genome that could improve our knowledge on host–pathogen interaction.