This paper presents CCD multicolour photometry for the old open cluster NGC 188. The observations were carried out as a part of the Beijing--Arizona--Taiwan--Connecticut Multicolour Sky Survey from 1995 February to 2008 March, using 15 intermediate-b
and filters covering 3000--10000 AA. By fitting the Padova theoretical isochrones to our data, the fundamental parameters of this cluster are derived: an age of $t=7.5pm 0.5$ Gyr, a distant modulus of $(m-M)_0=11.17pm0.08$, and a reddening of $E(B-V)=0.036pm0.010$. The radial surface density profile of NGC 188 is obtained by star count. By fitting the King model, the structural parameters of NGC 188 are derived: a core radius of $R_{c}=3.80$, a tidal radius of $R_{t}=44.78$, and a concentration parameter of $C_{0}=log(R_{t}/R_{c})=1.07$. Fitting the mass function to a power-law function $phi(m) propto m^{alpha}$, the slopes of mass functions for different spatial regions are derived. We find that NGC 188 presents a slope break in the mass function. The break mass is $m_{rm break}=0.885~M_{odot}$. In the mass range above $m_{rm break}$, the slope of the overall region is $alpha=-0.76$. The slope of the core region is $alpha=1.09$, and the slopes of the external regions are $alpha=-0.86$ and $alpha=-2.15$, respectively. In the mass range below $m_{rm break}$, these slopes are $alpha=0.12$, $alpha=4.91$, $alpha=1.33$, and $alpha=-1.09$, respectively. The mass segregation in NGC 188 is reflected in the obvious variation of the slopes in different spatial regions of this cluster.
