A base station and an uplink transmission method thereof are provided. The base station receives a plurality of first uplink transmission requests of a plurality of first wireless devices of a first group and a plurality of second uplink transmission requests of a plurality of second wireless devices of a second group in a first time interval, and approves one of the second uplink transmission requests. The base station receives second uplink transmission data of the second wireless device corresponding to the approved second uplink transmission request in a second time interval behind the first time interval. The base station receives the second uplink transmission requests of the second wireless devices corresponding to the unapproved second uplink transmission requests in a third time interval behind the second time interval.