A system for 3D online sports athletics includes: a cloud server and several head-mounted devices, wherein each of the head-mounted devices is paired with an intelligent terminal having a step counting function. A 3D scene virtualizing unit virtualizing a 3D online sports athletics scene to display the virtualized objects of all the head-mounted device users. A wirelessly connecting unit wirelessly connecting the intelligent terminal that it is paired with and the cloud server. A processing unit acquiring step counting data of the intelligent terminal and uploading the step counting data to the cloud server, and acquiring the step counting data of other head-mounted devices from the cloud server, driving the 3D scene virtualizing unit to display online the step counting data of each of the head-mounted device users in its 3D online running athletics scene, to realize sports athletics where a plurality of persons are simultaneously online.