Requests grooming in multi-layer architecture are realized by the joint optimization of IP and optical networks in optical interconnection of data center. Due to the burstiness of requests in intra-data center, burst requests are groomed to optical network, which leads to the longer delay of queue and establishment of light paths of requests. In order to reduce the establishment delay of optical paths, we propose a routing and spectrum assignment (RSA) algorithm with low delay, which separates the spectrum resource of fibers into spectrum layers, and multi threads can set up optical paths in different spectrum layers in parallel. The algorithm sets up colored virtual topology with the points and edges of establishing optical paths in every spectrum layer, and the topology is used as constraints of resource utilization in current spectrum layer. The colored and usable virtual topologies in spectrum layers are reconfigured according to transformations of establishing, unestablished and new arrived requests, which can adapt bursts of requests and promptly groom the requests swarmed in optical network. The simulation results demonstrate that the proposed algorithm can efficiently reduce the average delay, delay variance of establishment of optical paths and blocking probability of requests, and improve the rate of spectrum utilization and quality of experience (QoE).