The invention relates to a soft foundation reinforcing method of the existing railway or highway subgrade construction, which comprises the following steps of: constructing vertical chemical churning piles on the side slope and the slope toe of a subgrade to be reinforced; after the vertical chemical churning piles are cured, drilling a plug hole tilting toward the center line of a track and extending into the soft foundation in the side slope by using a drilling machine; and performing shock tube grouting in the plug hole. A sizing agent in the plug hole permeates towards the periphery of the foundation, the cured sizing agent, the vertical chemical churning piles and the foundation form an integrated reinforcing block, and thus, the reinforcement of the railway or highway subgrade construction is completed. Compared with the traditional single technique, the method has the advantages of no construction transition, no traveling interruption and no reinforcing dead zone by adopting the vertical chemical churning piles together with a shock tube grouting technique. The advantages of both the vertical chemical churning piles and the shock tube grouting technique are fully exerted, and the safety and the quality are controllable.