A smart phone with application of preventing children from being left in a car includes a speedometer, a data processing center, a phone speaker, a micro phone, a touch screen and an application of preventing children from being left in a car (application). the application is programmed as the follows: the speedometer detects car speed and reports the car speed data to the application, when the car speed increasing from first speed to second speed the application issues a [REMINDER] of “if has children in the car” to a driver, when the car speed decreasing from third speed to 0 speed, the application issues a [WARNING] of “must bring children out of the car” to the driver.