This article presents a design method for broadband digital phase shifter (PS). In this design, the phase-slope tuning properties of microstrip to slotline transition and λ/4 short-circuit microstrip-loaded slotline structure are researched and exposed. Equivalent transmission line models and a set of closed-form equations for both structures are presented for the purpose of revealing the mechanism of phase-slope tuning properties while predicting their transmission and matching characteristics. Design methods and specific design parameters for 11.25°, 22.5°, and 45° PSs are provided, for the case where two structures are used independently. Combining two structures for PS designs with larger phase shift range (PSR), such as 90° PS, has likewise been investigated to improve the flexibility of the proposed design method. In order to verify the correctness and practicality of the theoretical analysis, 11.25°, 22.5°, 45°, 90°, and 180° single-bit PSs and a 5-bit 360° broadband digital PS are designed and fabricated. From measurement, the operating bandwidth, return loss (RL), maximum root mean square (rms) phase error, and maximum rms amplitude error of this 5-bit PS are 2.55-4.5 GHz (55%), 10 dB, 4.5°, and 0.35 dB, respectively. The average insertion loss (IL) at the center frequency is 5.7 dB. The measured results demonstrate the usefulness of the proposed two structures with phase-slope tuning property for broadband PS design.