The passive space-borne atmospheric sounding spectrometer is a multi-mode hyper-spectral spectrometer and in a sun synchronous polar orbit. One of its main objectives is to observe the same atmospheric volume in both nadir and limb within one orbit, i.e. nadir/limb matching . In order to observe the same atmospheric volume in both nadir and limb, the method and time interval ΔT of nadir/limb matching are researched. According to the operation modes of the payload, satellite orbit and Earth’s rotation, the mathematical matching model is founded to study the method of nadir/ limb matching. Through calculation of Matlab and simulation of nadir/ limb matching in Satellite Tool Kit (STK), the time interval are intended to be 429 s and 430 s respectively. At last, considering Earth’s oblateness and orbit decay, combining with the two settings of nadir and limb scanning, and uniforming detection target area of nadir and limb, the final time interval is confirmed to be 430 s.