A water filter with alarm function for indicating blockage of a filter cartridge thereof and an alarm method are provided. The filter main body is provided with a filter cartridge and is arranged with a before-filtering water cavity. A filter upper cover covers the top of the filter main body. The before-filtering water cavity is accommodated with a floating-ball-type indicating apparatus, which comprises a floating ball, a connecting rod and a color scale. The connecting rod is mounted rotatably on an inner surface of the filter upper cover with the aid of a hinge support. The floating ball is arranged at the front end of the connecting rod. The color scale is also arranged on the connecting rod. A transparent visual cover that matches with the color scale is arranged on the upper surface of the filter upper cover.