The invention provides a method for reducing a magnesium-lithium ratio of salt lake brine. The method comprises the following steps of: introducing salt lake brine and ion sieve suspension liquid into an adsorption tank, stirring, regulating pH of mixed liquor, adsorbing lithium ions in brine by an ion sieve, moving out raffinate through a first film separating device, and feeding lithium ion sieve mixed liquor which is adsorbed to be saturated into a washing tank; under stirring, using pure water to wash magnesium ions in the washing tank, moving out raffinate through a second film separating device, and feeding the washed iron sieve suspension liquid into a desorption tank; introducing hydrochloric acid into the desorption tank, regulating pH of the mixed liquor under stirring, controlling certain desorption standing time, carrying out desorption regeneration of the iron sieve, collecting a lithium-containing desorption solution through a third film separating device, feeding the desorbed and regenerated ion sieve into an adsorption tank to recycle. The method realizes direct utilization of a nano ion sieve adsorbent, prevents adsorption performance from being reduced in a pelletizing process, and has the advantages of being simple in equipment, small in floor space and capable of realizing continuous operation.