A network resource allocating method for a residential gateway includes the following steps. A first downlink bandwidth is allocated to a residential gateway for process of a first service. A second downlink bandwidth is allocated to the residential gateway for process of a second service. A processing priority of the first service is higher than a processing priority of the second service. A first downlink rate of the residential gateway is detected when the residential gateway processes the first service. A second downlink rate of the residential gateway is detected when the residential gateway processes the second service. A bandwidth usage state of the residential gateway is determined according to the first and the second downlink rates. The second downlink bandwidth of the residential gateway is adjusted according to the bandwidth usage state such that the quality of service (QoS) of the first service or the second service is enhanced.