The invention discloses a nano starch filtrate reducer for drilling fluid and a preparation method thereof. The preparation method includes steps: S1, preparing nano starch; S2, preparing etherified and modified nano starch; S3, preparing a nano starch filtrate reducer coarse product; S4, purifying to obtain the nano starch filtrate reducer. The nano starch filtrate reducer can obviously lower normal-temperature API filter loss of basic mud and high-temperature high-pressure filter loss and is extensive in application range, and nano starch particles can be completely biodegraded, thereby being nontoxic and harmless to environment and excellent in environment protection performance. The filtrate reducer is reliable in preparation principle, controllable in reaction condition and low in cost of raw materials for preparation, and drilling engineering cost is lowered.