The invention discloses an on-line monitoring and controlling 
system and method for 
defrosting of an air conditioner. The on-line monitoring and controlling 
system comprises an air conditioner 
copper pipe, a dust guard, a filtering net, a plurality of 
infrared sensors for measuring 
radiation force of the air conditioner 
copper pipes, at least one 
thermocouple for measuring temperature of the air conditioner 
copper pipes, and a central control 
chip, wherein the central control 
chip is used for obtaining data returned by the 
infrared sensors and the thermocouples, and working out the actual 
emissivity of the surfaces of the copper pipes according to the Stefan-Boltzmann law so as to judge whether 
defrosting is needed or not and control a 
defrosting component to operate. The frosting condition is monitored based on the Planck law according to the principle that the value of difference between the copper 
pipe material 
emissivity and the 
frost emissivity is large, and the value of difference of the 
radiation power is large at the same temperature; hardware is improved little, and cost is low. The direct judgment strategy is adopted, monitoring is conducted in real time, and whether frosting exists or not is judged directly and precisely, and a defrosting 
signal can be emitted or interrupted in time; defrosting does not 
lag, so that the air conditioner keeps operating with the optimal performance, and the defrosting situation is judged accurately.