Azimuth multichannel spaceborne SAR system are able to overcome the limitation of minimum antenna constraint, so that it can yield high resolution and wide swath (HRWS) images. For this reason, this system is becoming research hotspots now. For azimuth multichannel spaceborne SAR system, gainphase errors between channels have a negative effect on the quality of imaging of system, so we must calibrate those errors. For rangevariant characteristics of phase error and negative effect on estimation caused by noise, utilizing Signal Subspace Comparison algorithm, we put forward a method of gainphase error estimation based on transmitter on the ground. Through suitable disposition oftransmitter,we can overcome rangevariant characteristics of phase error. Through controlling the power of transmitter, we can restrain the negative effect on estimation caused by noise. Finally, we validate it through simulation. It shows that we can solve the problem caused by rangevariant characteristics of phase error and the effect of noise by using a few transmitter. At the same time, in this way we can guarantee the precision of algorithm and it can also abates the amount of calculation.