From Singapore to tibet of China for 13 days, do I have to apply visa?

Hey, Singaporeans have free access to China for 15 days if they travel there. But I think you'd better apply one in advance in case something urgent happens and you can't extend your stay immediately. All foreigners who travel to Tibet have to apply for Tibet Travel Permit and join group tours arranged by qualified travel agencies. So I am afraid that you may not visit Tibet alone.