The invention discloses a transient electromagnetic method through casing water exploration tester. The transient electromagnetic method through casing water exploration tester comprises an iron core and an outer protective pipe sleeved on the outer side of the iron core, wherein an annular space is formed between the outer protective pipe and the iron core, the annular space is upwards communicated with the upper side of the iron core, and the lower end of the annular space is arranged between the outer protective sleeve and the iron core and is sealed by an O-shaped seal ring. A lower pressing cap is arranged on an annular stage formed on the upper end of the iron core, an upper pressing cap is arranged on the outer side of the lower pressing cap, a support block is arranged on the upper pressing cap, and a spring is arranged in a support block. The lower pressing cap is connected with an adapter joint at the upper side through an internal thread formed in the inner side of the lower pressing cap. A pressing cap is arranged on the annular stage formed on the lower end of the iron core, a support block is arranged on the pressing cap, and the spring is arranged in the support block. A guiding joint is arranged right below the iron core. The transient electromagnetic method through casing water exploration tester solves the problems that the existing cased hole completion production well has difficulty in exploring water and testing; conventional sine wave resistivity test is low in signal to noise ratio and can be affected by external environment to a great extent.