A transmission mechanism for switching output powers includes first to fourth transmission gears, first and second power-connecting members and a driving mechanism. The first power-connecting member coaxially disposed between the first and second transmission gears is moved axially, and connects the first and second transmission gears together to transmit power to the second transmission gear. The second power-connecting member coaxially disposed between the third and fourth transmission gears is moved axially and connects the third and fourth transmission gears together to transmit power to the fourth transmission gear. The driving mechanism synchronously drives the first and second power-connecting members to move axially to selectively connect the first transmission gear to the second transmission gear, or connect the third transmission gear to the fourth transmission gear. A scanning apparatus using the transmission mechanism is also disclosed.