The invention discloses a control method for a key size, which relates to the photolithographic process in the field of semiconductor. The control method comprises the following steps: providing a plurality of control pieces, and forming simulated substrates on the surfaces of the control pieces; coating chemical amplification photoresist on the simulated substrates of the control pieces, and carrying out exposure; carrying out the step of development; after the development, measuring the key size for the control pieces, and setting the measured size as a reference size; placing the control pieces in an environment in which wafers are produced and the storage and the transfer are the same for different time; and measuring the key size of the control pieces which are stored for different time, and establishing a coincidence relation curve of the key size and the storage time. The storage time corresponding to the part of the curve between the reference size and the maximum qualified size for producing the wafers is the safe storage time for producing the wafers. The control method finds out the safe storage time of the wafers through the coincidence relation between the key size after the development and the storage time. The control method has a shorter cycle and improves the production efficiency and ensures the yield of good products.