In order to solve the crosstalk problem and reduce the blocking rate, an adaptive threshold and spectrum first 渊AT-SF冤algorithm is proposed. The fiber cores in each group are grouped so that they are not adjacent, and a typical 7-core fiber is dividedinto three groups. The priority level of the third group of fiber cores is variable during the service arrival process.. At the sametime, the AT-SF algorithm introduces the frequency slot渊FS冤threshold parameter, and allocates the services greater than the FSthreshold to the first, second or third group of fibers, while the services less than or equal to the FS threshold are only allocated tothe third group of fibers. The simulation experiments are carried out in NSFNET and USNET networks, and the performance offirst match渊FF冤, 3D resource allocation渊3D-RA冤, path first渊aW-PF冤and conjugate gradient spectrum first渊CG-SF冤algorithmsare compared. The simulation results show that the AT-SF algorithm can obtain better blocking rate and crosstalk performancethan other algorithms when the network is under high load.