With the progress of high-capacity radio access networks, ultra-dense small cells are rapidly being deployed in urban areas. As a result, the deployment of a large number of optical fibers in urban areas becomes a severe issue. In this letter, we propose a hybrid fiber-terahertz (THz) mobile fronthaul system supporting flexible and high-order wireless signal transmission with the delta-sigma modulation. The photonic THz transmission is used as the seamless extension of fiber-based fronthaul in small cells. A 20Gbit/s digital fiber-THz fronthaul system is experimentally demonstrated to validate the proposed scheme, with 10km optical fiber transmission and 300GHz wireless relay. Carrier aggregation of up to 10 40MHz and 60MHz 5G-new radio (5G-NR) channels at the radio carrier frequency at 3.9GHz is reported. With the design of quantization noise suppressed delta-sigma modulation, the supported quadrate amplitude modulation (QAM) order is up to 16384 with the error vector magnitude (EVM) below 0.5%.