Abstract: Tapered finline array plays an important role in waveguide-based spatial power combiner. This paper presents a simplified model of tapered finline array by taking it as a tapered TE mode wave impedance transformer. The spectral domain admittance method is applied for deriving the propagation constant. Based on the small reflection theory, a new compact and broadband Hecken finline taper array is proposed. The structure is simulated and optimized by HFSS. By introducing slotline-to-microstrip line transition, the finline array in X-band is manufactured. The back-to-back test results of optimal 22 Hecken finline taper arrays consistent with theoretical values show that the return coefficient is less than -12 dB and the insert loss is less than 1 dB in X-band (8~12 GHz). This paper presents the method of analysis and design of the finline array, and provides a guideline for designing the waveguide-based power combiner. It serves as a promising circuit for power combining.