This paper puts forward a new algorithm for the secondary development of CATIA surface normal vector and coordinate transformation in VB language.Based on the direction cosine method and three special points on the axis of the new coordinate system, the algorithm ingeniously solves the instability of the function provided by CATIA for measuring angle in the secondary development of VB language, and avoids a series of calculations (such as calculating the direction cosine value by angle) directly using measuring angle, thus providing a strong guarantee for the correctness of CMM data source. Based on the research results, The author has successfully developed "CMM measurement point acquisition auxiliary software" with VB language and obtained the relevant software copyright.