To improve energy saving efficiency of Tianjin office buildings, we select high-rise office tower as an example and assess the effect of geometric factors on building energy performance, also identify the most energy-efficient geometric design. A prototype model is established based on survey data. We choose 6 geometric factors according to previous studies: plane aspect ratio (length to width), orientation, floor area, floor height, window-to-wall ratio and overhang depth. Then orthogonal method and the listing method are utilized to assign DesignBuilder energy simulation experiments. The purpose of minimize building energy consumption by optimizing the above listed factors is achieved. The effect of building fabric performance on result is also taken into account by setting 2 types of building fabric: conventional fabric and passive house fabric. Results show that floor height and window-to-wall ratio constitute two main related issues, followed by floor area, plane aspect ratio and orientation, and overhang depth. The optimal solution is characterized by plane aspect ratio of 2/1, orientation of south, floor area of 1 024 m², floor height of 3.6 m, window-to-wall ratio of 0.3 and overhang depth of 300 mm. The results can serve as a reference for passive design related to geometric factors in the practice of office building design in Tianjin.