The present application relates to a quick string-tying device including a first case body provided with a first bottom portion, a first open end, and a first sidewall extending and formed from the first bottom portion towards the first open end, the first bottom portion and the first sidewall forming a first accommodation space, the first bottom portion being provided with a first string-passing opening for two free ends of a string to pass therethrough, a slit formed on the first sidewall and extending from the first open end towards the first bottom portion, the slit being used to clamp the two free ends of the string which enter into the slit. The quick string-tying device is suitable for use on various items that require tying of a string, and can achieve quick tying and loosening of a string. A method for tying a string is also disclosed.