Rock mass quality rating index is the basis of rock slope stability evaluation. In revised Standard for engineering classification of rock mass

the calculation formula of quality index of slope engineering rock mass(QISERM) is added. In the formula there is a correction coefficient F3

which reflects the relationship between the dip angles of the main discontinuity and the slope. According to the standard

the value of F3 is determined by the difference between the dip angles of the main discontinuities and the slope. The paper discussed the flaw existing in the aforementioned method in determining F3

and gave the modification suggestions by applying the difference between the dip angle of the main discontinuity and the apparent dip angle of the slope in the dip direction of the main discontinuity. In addition

the QISERM of 13 357 examples were calculated using the two methods suggested by the standard and this paper

and then the difference of [BQ] between these two methods was analyzed. It shows that when the correction coefficient of discontinuity type and extension ? is equal to 0.6

the difference is less than 40 and 78.3% of them is less than 5; when ? is equal to 1.0

the difference is less than 60 and 68.7% of them is less than 5. The method proposed in this paper is more complete and more accurate than the new standard method in theory

and it can provide reference for the calculation of QISERM.