The invention discloses a thermal desorption, gas-phase separation and thermal cracking system based on ethylation derivation of methyl mercury, comprising a thermal desorption part, a gas-phase separation part, a thermal cracking part, a gas flowmeter, a Teflon FEP pipeline and a pressure regulating and temperature control switch chamber, wherein the thermal desorption part comprises a thermal desorption heater strip switch, a thermal desorption heater strip time control switch, a thermal desorption heater strip pressure regulator, a pressure regulating knob of the thermal desorption heater strip pressure regulator, a thermal desorption heater strip and a Tenax adsorption pipe; the gas-phase separation part comprises a heater jacket, a heater jacket switch, an OV-3 packed column and a heater jacket temperature control plate; and the thermal cracking part comprises a thermal cracking heater strip switch, a thermal cracking heater strip time control switch, a thermal cracking heater strip pressure regulator, a pressure regulating knob of the thermal cracking heater strip pressure regulator, a thermal cracking heater strip and a quartz pipe. The system realizes the aim of integration of thermal desorption, gas-phase separation and thermal cracking devices, and simultaneously, each part can be individually operated, thereby saving the analysis time, having low manufacturing cost and greatly reducing experiment cost.