Design integrates concepts and solves problems. Crowdsourcing design imports vast knowledge and produces creative ideas. It publishes design tasks, collects dozens of contributors' ideas and reward the best. Contributors in crowdsourcing design work individually when generating ideas. However, those who collaborate could make better use of crowd's knowledge, which might produce ideas of higher quality. By analyzing the advantages and disadvantages of crowdsourcing design, this article proposes a collaborative crowdsourcing design method that integrates crowd's sketching processes. This method uses a tree to arrange crowd's ideas and enable flexible modifications of prior ideas. A demonstration system named Sketchfans is developed, and the results of a crowdsourcing sketching experiment using this system are presented. The experimental results validate that this method is effective; participants rely heavily on the idea tree for inspiration, and the best ideas appear around the ends of branches on the idea tree. Moreover, participants displayed unique development patterns. They often developed high-quality ideas from initial ideas that were regarded as poor quality. The demonstration system Sketchfans, supports this method well. Finally, we optimize Sketchfans based on analysis of participants' activities and feedback.