We present a simple system for realizing single-shot ultrafast sequential imaging based on spatial multiplexing in-line holography. In this system, we propose to combine a specially designed mini-reflector delay-line array with digital in-line holography. The former including a group of adjustable mini-reflectors can easily generate an array of probe sub-pulses that can be controlled independently in the propagation direction and time delays. The object beams formed by the different sub-pulses will propagate and fall on different recording regions of the image sensor to generate a single-shot spatial-multiplexing in-line hologram. The geometry of the digital in-line holography can simplify the complexity of the system and enable complex amplitude imaging. In addition, the time resolution of this system is limited only by the pulse duration, which allows this system to study the dynamic processes with the femtosecond order. In an experiment about the laser-induced air plasma, our proposed system achieves nine frames sequential holographic images with the frame rate of 7.5 trillion frames per second (Tfps).