Remote-target detecting and isolation between transmitter and receiver are the problems that need to be solved when FMCW SAR is applied in high-altitude slow-speed airborne platform. A signal model of inter-pulses interrupted frequency-modulated continuous waveform (IPIFMCW) is proposed in consideration of the features of high-altitude slow-speed airborne platform. The imaging of high-altitude slow-speed airborne SAR is realized through the gating signal with different carrier frequency to modulate the transmit signal to obtain long inter-pulses interval and shifting the mixing echo of sub-pulse in fast time domain to get wide-band signal. Compared with the FMICW theory, the results show that IPFMICW not only overcomes the problem of isolation between transmitter and receiver, but also fulfills the need of remote-target imaging for high-altitude airborne SAR and avoids range ambiguity. Finally, the imaging of high-altitude slow-speed airborne SAR based on IPIFMCW signal is realized through frequency scaling algorithm (FSA).