Magnetic induction (MI) communication is an effective underground emergency communication technique after disasters such as landslides, mine collapses, and earthquakes, due to its advantages in mediums such as soil, concrete, and metals. Based on channel state information (CSI), magnetic beamforming can significantly improve the performance of MI communication. However, in post-disaster underground communication, channel estimation may suffer from errors due to factors such as complex environmental interferences. Taking channel estimation error into account, we formulate a beamforming optimization problem for multi-user MI underground emergency communications, which aims to minimize the power consumption under the constraints of sum rate and signal to interference plus noise ratio (SINR) of each user. Based on the worst-case optimization criterion and the S-procedure, the non-convex optimization problem is transformed into convex and solved. Numerical results show that the proposed robust beamforming scheme can effectively enhance communication reliability and effective throughput in the presence of channel estimation errors.