Abstract: To explore the optimal delay time between the blast holes,high precision experiment device and technology are adopted,including high speed photography,dynamic strain test method,digital electronic ignition system and others.Considering the factors from new free surface forming time and the residual mount of stress wave,investigation is made through concrete slope model of single hole blasting of high speed photography and hole by hole blasting dynamical strain test.The results show that:when the delay time between holes is 5 ms,the stress wave superposition between two holes is good,but the good free surface is not formed at the later hole；When the delay time between holes is 10 ms,the formation of cracks in 10 mm above is ensured,and also the subsequent hole still have a larger residual stress existing in the medium.It is conducive to the formation of good crushing effect；When the delay time between hole is 15ms,the residual stress generated by the front hole almost disappeared；When the delay time between holes is above 20 ms,blasting of the latter hole is not affected by the front hole.After comprehensive consideration,it is suggested that the optimal delay time between holes ranges from 10 to 15 ms in hole-by-hole millisecond blasting.

Key words: Precise time delay, High speed photography, Dynamic strain test method, Digital electronic initiation system