In the past twenty years, research heat and market demand for image sensor technologies have grown rapidly, and image technologies are undergoing the transition from Charge-Coupled Device (CCD) to Complementary Metal Oxide Semiconductor(CMOS). Since there are a large number of differences between the two technical difficulties, a lot of research directions have changed, but improving the transmission efficiency of charge in pixels has always been one of the most important research directions. The key point of CMOS Image Sensor (CIS) pixel design is the transfer of photo-generated charge in photo-diode. Especially in applications requiring large pixels, such as medical imaging and astronomical imaging, limited signals need to be detected at high speed, which requires high transmission efficiency of charges in photodiode.Charge transfer is a complex process affected by many factors. It is usually divided into drift, diffusion and self induced drift.In order to improve the charge transfer speed in CIS pixel, a large number of solutions have been proposed. Generally, it can be divided into two directions: increasing the bias electric field externally or generating the potential gradient internally. The method of adding an external bias electric field, has great scenic limitations. The methods of forming internal potential gradient in a photo-diode can be roughly divided into two directions: designing the special shape and manufacturing gradient doping. Designing specific diode shapes, are relatively complex, and the formation of a built-in electric field is actually the cost of reducing the filling factor, which reduces the space utilization and virtually increases the cost of actual production. Manufacturing gradient doping^{are lack complete theoretical analysis, which will take a lot of time to adjust parameters in practical operation, and there is no rigorous reference value. Therefore, there is a great need for a complete and relatively refined analysis method to provide theoretical support for the design and research of gradient doping.Based on Fick's law and electron drift theory, this paper deeply analyzed the whole process of ion implantation and diffusion and proposed an efficient implementation method of arbitrary gradient doping distribution. Taking the manufacturing of linear-gradient doping distribution as an example, the correctness of this method is verified by simulation calculation and process simulation. By this method, the process parameters such as ion implantation energy, implantation dose and the time required for thermal diffusion can be accurately calculated. Then, according to the target doping distribution, a lithography mask with a specific structure can be designed. The doping region with an arbitrary distribution function can be fabricated by only one ion implantation.A 5 μm CMOS image sensor pixel with gradient doped photo-diode is designed by this method. A set of control experiments were set up with a pixel designed by traditional technology. Compared with pixels designed by traditional technology, the experimental results show that the transmission efficiency of photo-generated charge in the pixel with gradient doped photo-diode is significantly improved. The method proposed in this paper can greatly improve the efficiency of researching gradient doping. In addition, this method can also be used in other fields, such as studying the diffusion coefficient of specific impurities in substrate materials, determining some physical properties of new materials, etc.}