Can the family doctor of Dongguan social security card cancel the contract online?

You can't. According to official website of Dongguan Social Medical Security Bureau, the way for family doctors to sign social security cards is to cancel their contracts at the community health service center during working hours once a year (the contract signing period is 1 year), and then renew their contracts with other doctors or not. If residents confirm that they want to change the contract, they should contact the original signing team to apply. After the team is approved, it can cancel the contract in the social rehabilitation center and then re-sign other family doctors.