The invention relates to a dustproof cap for lock of oil tank. Fixture seat is set on the ring seat of lock of oil tank and action mechanism, which can open the cap of oil tank, is set on the fixture seat. The dustproof cap is covered on the fixture seat and accommodating space is set in the dustproof cap. The accommodating space comprises upper shield piece, side shield piece and lower shield piece so that eyewinker is prevented invading fixture seat and uneasy action of action mechanism generated by invading of eyewinker is avoided.