A method for preparing chlorine comprises the following steps that S1, a refined saturated sodium chloride solution is added into a preheater to be preheated with the preheating temperature of 47-63 DEG C; S2, the preheated saturated sodium chloride solution is transferred into an anode chamber of an electrolytic bath, distilled water is introduced into a cathode chamber, a power supply is turned on to supply power, acid is added into the anode chamber when the current of a circuit reaches 5.3 KA, and the pH value of the cathode chamber is maintained at 2-3; S3, the electrolytic bath is heated to 58-67 DEG C, and heat preservation and energizing reaction is conducted; S4, a vacuum pump at the electrode end of the anode chamber is started for gas pumping, and the pressure of the anode chamber is maintained at 47.35*10<3> Pa-53.41*10<3> Pa; and S5, pumped-out gas is conveyed to a dryer provided with a drying agent through a coil pipe to be dried, and the dried chlorine is conveyed into a storage tank to be stored. By adoption of the method, the output of the chlorine is increased, the chlorine is also effectively prevented from dissolving in water again, generated steam is small, and improvement of the purity of the chlorine is facilitated.