A method for implementing a high-resolution wide-swath spaceborne Synthetic Aperture Radar, SAR, system, comprising steps of: adopting, in azimuth of a spaceborne SAR, a Multi-Input Multi-Output, MIMO, sliding spotlight mode for transmitting, by N sub-apertures, N chirp signals of different carrier frequencies, such that sub-band echoes received by the multiple sub-apertures are separated; and preprocessing, in azimuth, the separated sub-band echoes to resolve the back-folded Doppler spectrum caused by beam steering and a multi-channel structure of an antenna; performing range sub-band synthesis on azimuth preprocessed signals to obtain a synthesized signal with a bandwidth of N times of a bandwidth of a single transmitted chirp signal; and performing, using a Range-Migration Algorithm,RMA, residual focusing of the synthesized signal to obtain a final radar image with a high resolution wherein the step of performing, using an RMA, residual focusing of the synthesized signal comprises steps of: transforming a synthesized wideband signal to a two-dimensional frequency domain; multiplying, in the two-dimension frequency domain, the synthesized wideband signal by a reference function to obtain a roughly-focused radar signal; and performing a STOLT mapping on the roughly-focused radar signal to obtain the final radar image, characterized in that the reference function H RFM ( f τ, f η) in the RMA is expressed as: H RFM f τ f η = exp j ⁢ 4 ⁢ π ⁢ R ref ⁢ f c + f τ c ⁢ 1 - c 2 ⁢ f η 2 4 ⁢ v s 2 ⁢ f c + f τ 2 ⋅ exp jπ ⁢ R s ⁢ f η 2 ⁢ c 2 ⁢ v s 2 ⁢ f c + f τ, wherein f τ is a range frequency, f η is an azimuth frequency, R ref is a nearest slant range between the centre of the imaging scene and the antenna, f c is a central carrier frequency, c is the speed of light, v s is a velocity of a platform, j is an imaginary unit, π is a ratio of the circumference of a circle to its diameter, R s is a nearest slant range between the antenna and a centre of rotation, and the second exponential term is for compensating an additional phase introduced by azimuth bulk compression, which is not included in a reference function in the strip-map mode. adopting, in azimuth of a spaceborne SAR, a Multi-Input Multi-Output, MIMO, sliding spotlight mode for transmitting, by N sub-apertures, N chirp signals of different carrier frequencies, such that sub-band echoes received by the multiple sub-apertures are separated; and preprocessing, in azimuth, the separated sub-band echoes to resolve the back-folded Doppler spectrum caused by beam steering and a multi-channel structure of an antenna; performing range sub-band synthesis on azimuth preprocessed signals to obtain a synthesized signal with a bandwidth of N times of a bandwidth of a single transmitted chirp signal; and performing, using a Range-Migration Algorithm,RMA, residual focusing of the synthesized signal to obtain a final radar image with a high resolution wherein the step of performing, using an RMA, residual focusing of the synthesized signal comprises steps of: transforming a synthesized wideband signal to a two-dimensional frequency domain; multiplying, in the two-dimension frequency domain, the synthesized wideband signal by a reference function to obtain a roughly-focused radar signal; and performing a STOLT mapping on the roughly-focused radar signal to obtain the final radar image, transforming a synthesized wideband signal to a two-dimensional frequency domain; multiplying, in the two-dimension frequency domain, the synthesized wideband signal by a reference function to obtain a roughly-focused radar signal; and performing a STOLT mapping on the roughly-focused radar signal to obtain the final radar image, characterized in that the reference function H RFM ( f τ, f η) in the RMA is expressed as: H RFM f τ f η = exp j ⁢ 4 ⁢ π ⁢ R ref ⁢ f c + f τ c ⁢ 1 - c 2 ⁢ f η 2 4 ⁢ v s 2 ⁢ f c + f τ 2 ⋅ exp jπ ⁢ R s ⁢ f η 2 ⁢ c 2 ⁢ v s 2 ⁢ f c + f τ,