Quantum technology establishes a foundation for secure communication via quantum key distribution (QKD).In the last two decades,the rapid development of QKD makes a global quantum communication network feasible.In order to construct this network,it is economical to consider small-sized and low-cost QKD payloads,which can be assembled on satellites with different sizes,such as space stations.Here we report an experimental demonstration of space-to-ground QKD using a small-sized payload,from Tiangong-2 space lab to Nanshan ground station.The 57.9-kg payload integrates a tracking system,a QKD transmitter along with modules for synchronization,and a laser communication transmitter.In the space lab,a 50 MHz vacuum + weak decoy-state optical source is sent through a reflective telescope with an aperture of 200mm.On the ground station,a telescope with an aperture of 1200mm collects the signal photons.A stable and high-transmittance communication channel is set up with a high-precision bidirectional tracking system,a polarization compensation module,and a synchronization system.When the quantum link is successfully established,we obtain a key rate over 100 bps with a communication distance up to 719km.Together with our recent development of QKD in daylight,the present demonstration paves the way towards a practical satellite-constellation-based global quantum secure network with small-sized QKD payloads.