Two types of weak grating sensors with different packaging methods were developed using the weak grating strain monitoring technology. The strain of the steel beam after loading is measured and compared with the distributed optical fiber sensor. The experimental results show that the strain monitoring results obtained using weak grating agree well with those obtained using distributed optical fiber and theoretical calculation. The error between the calculated deflection of the steel beam and measured and theoretical values of the dial indicator is within 10%. The horizontal static load test results of an offshore steel pipe pile show that the deformation of the pile is mainly within the range of 60 m below the sea level. The deflection of steel pipe piles increases with the increase of load and decreases with the increase in pile depth. After 50 m buried depth, the steel pipe pile is almost no longer affected by horizontal thrust, and the deflection value is 0. In the horizontal static load test, the average error between the monitored deflection and actual deflection is 16 mm. Through the indoor model and practical engineering application, the feasibility and accuracy of weak grating in steel structure monitoring are proved, which provides technical support for similar projects.