Synthetic dimensions in photonic structures provide unique opportunities for actively manipulating light in multiple degrees of freedom. Here, a dispersive waveguide under the dynamic phase modulation is theoretically explored, which supports single pulse manipulations in the synthetic (2+1) dimensions. Compared with the counterpart of the conventional (2+1) space-time, temporal diffraction and frequency conversion in a synthetic time-frequency space are demonstrated while the pulse evolves along the spatial dimension. It is found that a rich set of pulse propagation behaviors can be achieved by introducing the effective non-uniform gauge potential for photons in the synthetic time-frequency space with the control of the modulation phase, including confined pulse propagation, fast/slow light, and pulse compression. With the additional nonperiodic oscillation subject to the effective force along the frequency axis of light, this work provides an exotic approach for actively manipulating the single pulse in both temporal and spectral domains, which shows the great promise for applications of the pulse processing and optical communications in integrated photonics.