Phase demodulation from carrier-frequency fringe patterns is the core of many optic measurements. We propose spatial quasi-phase-shifting technique by expressing the fringe signal in the frequency-modulated form, which requires only one frame fringe pattern for instantaneous and dynamic measurements. In an area smaller than a fringe period, there substantially exists an approximately constant phase shift between spatially adjacent sample points. The technique is capable of demodulating the phase with such intra-frame phase shifts, which makes the instantaneous and dynamic measurement possible. The technique implements demodulation within three spatially adjacent neighbors, achieving spatial localization as good as a several-point level. Both numerical simulation and experiment are presented to verify its performance.