The invention provides an on-line separation device with a changeable degassing amount for gases in insulating oil and a wall-mounted analysic system, and belongs to the automation field of electric power systems. The on-line separation device comprises an intelligent oil circulation module used for oil sample circulation, and an oil-gas balancing module used for balancing an oil sample and a characteristic gas. The on-line separation device has the advantages that no oil mist can be generated during the oil-gas balancing process; the on-line separation device can be stopped at any time; the number of components of the whole system is less; automatic control is realized via a control module; the degassing amount can be adjusted freely according to the volume of a gas analysis module; sweeping and replacing of the whole system are avoided during secondary detection; the oil-gas separation time is greatly shortened; the detection efficiency is improved.