The invention relates to a method for storing image data in an IOS (Iphone Operation System) by using an external memory, wherein the method comprises the following steps: acquiring original image data outputted by a camera in the IOS, transmitting to a first coding unit, and processing by the first coding unit; using the first coding unit to convert the transmitted image data to a standard video file convenient to store, and outputting the standard video file; calling a first lookup function and a second write function in the system to write the file outputted by an encoder to the external memory. The invention further relates to a device for implementing the method. Implementing the method and the device for storing the image data in the IOS by using the external memory has the following beneficial effects that the image data in the external memory are more rapidly and much easier to store.