Abstract: In order to establish the spatial position-reference accurately，and aiming at the characteristics of multi-period control network underground，the variable parameters sequential adjustment model of underground control network with constrained condition is presented.Firstly，based on the virtual observation law and the least square method，the sequential adjustment model of underground control network is deduced.Considering the gyro direction edges as strong sides，the constrained conditions can be converted into the observation equation of the variable parameters sequential adjustment based on infinite weight theory.The new model can be used for processing gyro-side observation as constraint condition.Also，it can be employed to calculate the gyro-side observation as general observation values.Then，in order to avoid the ill-condition of normal equation，the reasonable values of weight coefficient are discussed.Besides that，the rationality of the model is verified by a project case.The results show that:①when the weight coefficient m=1，the maximum difference between adjusted value and true value of gyro azimuth is 5.1″，with the increasing of weighting coefficient m, the difference decreases rapidly; ②when m=10，the difference is less than 0.1″;③when m=20，the adjusted values of gyro-side azimuth and coordinates are consistent with the true value within the scope of effective number of bits，the azimuth difference is less than 0.01″，and the coordinates of the difference is less than 0.03 mm.The adjustment model proposed in this paper has the characteristics of simple idea,easy programming and no preliminary observation data needed，it is applicable to the integrated adjustment of the underground multi-period control network.

Key words: Constrained condition, Underground control network, Sequential adjustment, Gyro directional edge, Infinite weight