A blow with yellow slurry appeared in the borehole of some hydropower station foundation when curtain grouting was conducting, and grout run out largely. According to the data of the deep collecting well in the workshop foundation, it was found that a weak interlayer with thickness of more than one meter exists in the underlain layer. So the construction was stopped.To find the effect on the foundation stability caused by distribution range and properties, twelve grouting boreholes were used to do the ultrasonic testing. Rock core was used for the single axis compression strength and ultrasonic wave velocity testing, then foundation quality was estimated and weak interlayer thickness and their distribution was revealed. The grouting effect is estimated by comparing ultrasonic wave velocity in grouting order Ⅰ with that of grouting order Ⅱ,finally, it provides important basis for foundation quality check and acceptance.