In this study, we designed and demonstrated the performance of a multi-antenna remote differential monitoring system based on a single GNSS-over-fiber architecture. In this system, multiple GNSS signals are received by remote antennas through a microwave photonic link and are then transmitted to local end points. To enable fine positioning, we established a double differential model equation for use between the carrier phase of each remotely received signal and the reference GNSS signal, with the help of the time division mode using a high-speed optical switch. In our experiment, we established a 10 km microwave photonic link among three remote monitoring points. We estimate the resulting positioning accuracy to be within several millimeters and we obtained a response time of less than 10 ms. Compared with traditional single-antenna schemes, our designed system has significant advantages with respect to coverage area, real-time response time, and the performance cost of large-scale monitoring at no cost to the positioning accuracy. As such, this system will find important applications for the monitoring of large-scale civil engineering and natural environments.