import numpy as np
import cv2 as cv
from matplotlib import pyplot as plt

img = cv.imread("raw.png")
img_rgb = cv.cvtColor(img,cv.COLOR_BGR2RGB)
gray_img = cv.cvtColor(img_rgb, cv.COLOR_RGB2GRAY)

plt.subplot(121),plt.imshow(img_rgb),plt.title('Original')
plt.xticks([]), plt.yticks([])
plt.subplot(122),plt.imshow(gray_img),plt.title('Grayed')
plt.xticks([]), plt.yticks([])
plt.show()

解决方法：

plt.imshow() 中加上cmap='gray'

plt.subplot(122),plt.imshow(gray_img, cmap='gray'),plt.title('Grayed')