It is significant to estimate the growth of crops during their growth stages rapidly and non-destructively, the spectral analysis technology could satisfy the requirement and it is one of the important techniques to support in the precision agriculture management. In the article, a system was developed for estimating the growth of crops in greenhouse, which was based on spectrum and WSN (wireless sensor network) technology. The system consisted of three parts: the intelligent sensor nodes, intelligent gateway and the software in remote server. The ZigBee wireless communication modules were embedded in the intelligent sensor nodes and gateway to create WSN. The intelligent sensor nodes contained four optical channels, which allowed these instruments work at the wavelengths of 550, 650, 766 and 850 nm. They were used to collect spectral data and transmit to the intelligent gateway by ZigBee wireless network. The intelligent gateway worked as a coordinator of the whole wireless network waiting for sensor nodes to join in and receiving all the spectral data from sensor nodes, and transmitting the data to remote server by GPRS. The software in remote server analyzed the spectral data, calculated the NDVI (Normalized Difference Vegetation Index), by which, the growth of the plants can be detected effectively. In order to test the effectiveness and stability of the system, the strawberry of greenhouse was detected. Several experiments were carried out in the greenhouse of ZhuoZhou Experiment Station in Hebei. Results showed that the canopy spectral data of strawberries could be acquired by the intelligent sensor nodes stably, and the intelligent gateway could connect the sensor nodes and the remote server effectively. The spectral data can be received by the remote server normally as well. The system provides a kind of technical support and theoretical basis for crop growth estimation in greenhouse.