The invention provides an advanced water detecting method of a subsea tunnel, comprising the following steps: opening a service tunnel which is parallel with two main tunnels between the two main tunnels as an advanced parallel heading thereof; making use of a drill jumbo for drilling simultaneously to form water detecting holes on each circular working face of the main tunnels and the service tunnel before explosion, wherein each working face comprises at least one water detecting hole; digging forwards continually if no seepage exists in the water detecting hole; and grouting in cranny at once to plug the seepage if seepage appears, then organizing to do explosion and other steps. The advanced water detecting method of the subsea tunnel provided by the invention sets an advanced parallel service tunnel between the two main tunnels according to the feature that the construction of the subsea tunnel in the fault fracture zone has relative high risk, uses the status of the ground water detected in front of the working faces of the main tunnel and the service tunnel to confirm the following construction project, thereby avoiding risks and ensuring the construction safety of the subsea tunnel.