A colloidal gold lateral flow (LF) strip reader based on CMOS image sensor, characterized by ultrasensitive and high repeatable, is developed. As gold nano-particles absorb green light strongly, green LEDs are used as the light source in the reader. In order to get the best illumination uniformity, gray value distribution of blank strip image is used to adjust the location and intensity of LEDs. CMOS control parameters are specified to get the most suitable image quality for image analyzing by self-defining parameters. Blank strip is used to calibrate the background signal. The quantitative result is derived from the ratio of integral reflection optical density of T line signal to C line signal. LF strips of cardiac troponin I (cTnI) with standard mass concentration gradients from 0.25 ng/ml to 64 ng/mL are detected. The 4-PL quantitative model is used to fitting standard working curve, whose correlation of coefficient (R2) is more than 0.99. The coefficient of variation (CV) of 20 times repetitive experiments of 0.25 ng/mL is 2.790%, and 64 ng/mL is 0.134%. The functional sensitivity (FS) is better than 0.25 ng/mL. The reader is sensitive, stable, simple, and suitable for field detection.