The invention discloses a foamless cleaning agent as well as a preparation method and application of the foamless cleaning agent. The cleaning agent comprises coconut fatty acid diethanolamide, n-decyl glucoside, polydimethylsiloxane, oleic hydroxyethyl imidazoline, isothiazolinone, potassium acetate, xanthan gum, paraffin oil and water. Oleic hydroxyethyl imidazoline in the foamless cleaning agent disclosed by the invention has a corrosion inhibition effect, so that a down-hole casing and an oil pipe can be subjected to corrosion protection when being cleaned, and the service life is prolonged; isothiazolinone has a bactericidal effect; the xanthan gum can improve the viscosity of the cleaning agent, so that the suspending capability is provided for solid-phase separation in a cleaning process, and a requirement of carrying a high-density solid-phase substance cleaned in a downhole is met; and meanwhile, the xanthan gum has a spatial net structure, so that the prepared cleaning agentproduct is more stable.