A roadside parking safety prompting system is mainly composed of an ultrasonic sensor, a controller and a range display which are sequentially connected. When a car is operated by a driver to back up for conducting roadside lateral parking, the system is started, whether the distance between tires and curbstone is larger than the safe distance or not is monitored in real time, and when the distance between the tires and the curbstone is smaller than the safe distance, an alerter is driven to alarm.