This paper presents MobiScatter, which enhances capacity of CSS based backscatter networks for accommodating drone-carried access points (APs). CSS based backscatter design has favorable features including long range and high concurrency. However, the concurrency of the network can be reduced by 34% when the drone-carried AP is introduced due to the mobility and fast fading. In order to maintain high concurrency, MobiScatter presents a series of new designs. In particular, we propose to enhance concurrency of the CSS based backscatter network with symmetric upchirps and downchirps, which neutralizes the impact of imperfect frequency orthogonality. Then, to mitigate the impact of fast fading on decoding, we present a novel half-period chirp modulation scheme for crossed chirps. Finally, we provide a power management method for tags by controlling transmitting time of chirps. We construct a MobiScatter prototype, which contains a drone-carried AP implemented with a mobile USRP and 200 tags. We deploy those tags in an area of $200 m\times 180 m$ on a meadow. Experimental results show that MobiScatter can support 160 concurrent backscatter transmissions when the AP moves at $15 m/s$ .