This paper describes the process of proving the sole existence of an orthographic projection for a 3-dimentional geometry based on the projection theorem in the optimal theory, and proposes a new method of calculating the orthographically projective transformation of an object. It includes: ①An orthographic projection of a 3-dimentional geometry is defined again with the mathematical concepts; ②In order to apply the projection theorem, a series of propositions related to the orthographic projection are proved sequentially, and then the sole existence of an orthographic projection for a 3-dimentional object is strictly testified and the orthographically projective transformation of an object is found with respect to the Fourier series; ③The advantages of the method of calculating the orthographically projective transformation are concluded.%拟应用最优化理论中的投影定理，从理论上严格证明物体平行正投影的存在和唯一性，并在此基础上建立一个全新的平行正投影计算方法.内容包括：①应用数学语言阐明三维物体平行正投影的含义；②应用最优化理论中的相关理论依次论证与投影定理有关的几个命题，并在此基础上严格证明物体平行正投影的存在和唯一性，以及利用傅里叶级数形式建立平行正投影计算式；③简要分析这种计算方法的特点.
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