The remarkable improvement of sensing and processing hardware performance makes the non-contact, on-line, high accuracy and high speed measuring possible by means of linear CCD. The goal of this paper is to describe the key questions on designing the cone diameter detecting system by linear CCD. This paper first describes the detecting scheme. In particular, the factors affecting accuracy, such as resolution, voltage difference between neighbor pixels, detecting rate and so on, are analyzed, and the quantitative estimation equations are provided. CCD takes on the photoelectric translation and measuring component double functions, so the waveform of CCD output signal and the affecting factors merit deep discussion. On the basis of discussion, several principles that can be used to improve the boundary recognition accuracy are presented.