Implantable neuroprostheses require stimulators with high channel counts and mechanical flexibility. Organic thin-film transistor (OTFT), an essential building block for flexible circuits and system, is a promising candidate. However, the development of photolithographic OTFTs for complete bioelectronic system integration remains a challenge, due to their limited yield and uniformity. This paper reports a 4-mask photolithographic OTFT circuit integration technology, which shows a high device yield of 100% (50/50) and small device variation in threshold voltage of 0.64 V and in mobility of 4.9%. Using a device-circuit-system co-design approach, we demonstrate an active-matrix neurostimulation array comprised of 1024 pixels of a 4T1C stimulation circuits, in which independent stimulation intensity levels can be programmed and current stimulus at all channels can output simultaneously. The electrical function of the complete neurostimulation system is verified, showing a small variation of 15.59% for the output stimulation currents among pixels. This OTFT-based neurostimulation system provides a potential solution for the next-generation neurostimulators with high channel counts and mechanical flexibility.