In this article, a type of a wideband continuously tunable phase shifter (PS) with phase slope tunability is proposed. It is designed based on the theory of the wideband multimode filter. By drawing the phase performance design graphs, phase performance with a specific insertion phase and a phase slope is mapped into the corresponding magnitude filtering performance. Proper magnitude states are then chosen to construct the required phase performance. By virtue of the tunable phase, tunable phase slope, and wideband operation, the proposed PS constructs a continuously tunable PS with a wide operation band, large phase shift range (PSR), and low in-band phase error by cascading switched-line PSs. From measurement, this structure is exhibited to obtain 0°-360° phase shift in a wide fractional bandwidth of 26.1%, while maintaining a low in-band phase error smaller than ±5.5°.