An alarm method for insufficient storage space of a network storage system is provided. The method includes the following steps. Preset a time alarm threshold value according to a current operation state of the system when allocating storage spaces to users. Estimate a time that written data fully occupy a current remaining space according to a data writing speed to the storage space, a current idle memory of the system, total dirty data in a cache of the system, and the current remaining space in the storage space. Compare the estimated time value and the time alarm threshold value. And, send alarm information when the estimated time value is smaller than the time alarm threshold value. By setting the alarm threshold value of a time concept, a timely alarm of insufficient storage space is realized, which enhances the effective management of the storage space.